Types of Networks
LANs (Local Area Networks)
A network is any collection of independent computers that communicate with one another over a shared network medium. LANs are
networks usually confined to a geographic area, such as a single building or a college campus. LANs can be small, linking as few as
three computers, but often link hundreds of computers used by thousands of people. The development of standard networking
protocols and media has resulted in worldwide proliferation of LANs throughout business and educational organizations.
WANs (Wide Area Networks)
Wide area networking combines multiple LANs that are geographically separate. This is accomplished by connecting the different
LANs using services such as dedicated leased phone lines, dial-up phone lines (both synchronous and asynchronous), satellite links,
and data packet carrier services. Wide area networking can be as simple as a modem and remote access server for employees to dial
into, or it can be as complex as hundreds of branch offices globally linked using special routing protocols and filters to minimize the
expense of sending data sent over vast distances.
Internet
The Internet is a system of linked networks that are worldwide in scope and facilitate data communication services such as remote
login, file transfer, electronic mail, the World Wide Web and newsgroups.
With the meteoric rise in demand for connectivity, the Internet has become a communications highway for millions of users. The
Internet was initially restricted to military and academic institutions, but now it is a full-fledged conduit for any and all forms of
information and commerce. Internet websites now provide personal, educational, political and economic resources to every corner of
the planet.
Intranet
With the advancements made in browser-based software for the Internet, many private organizations are implementing intranets. An
intranet is a private network utilizing Internet-type tools, but available only within that organization. For large organizations, an intranet
provides an easy access mode to corporate information for employees.
MANs (Metropolitan area Networks)
The refers to a network of computers with in a City.
VPN (Virtual Private Network)
VPN uses a technique known as tunneling to transfer data securely on the Internet to a remote access server on your workplace
network. Using a VPN helps you save money by using the public Internet instead of making long–distance phone calls to connect
securely with your private network. There are two ways to create a VPN connection, by dialing an Internet service provider (ISP), or
connecting directly to Internet.
Categories of Network:
Network can be divided in
to two main categories:
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Peer-to-peer.
Server – based.
In peer-to-peer networking there are no
dedicated servers or hierarchy among the
computers. All of the computers are equal
and therefore known as peers. Normally
each computer serves as Client/Server and
there is no one assigned to be an
administrator responsible for the entire
network.
Peer-to-peer networks are good choices for
needs of small organizations where the
users are allocated in the same general
area, security is not an issue and the
organization and the network will have
limited growth within the foreseeable future.
The term Client/server refers to the concept
of sharing the work involved in processing
data between the client computer and the
most powerful server computer.
The client/server network
is the most efficient way
to provide:

Databases and management of
applications such as
Spreadsheets, Accounting,
Communications and
Document management.
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
Network management.
Centralized file storage.
The client/server model is basically an
implementation of distributed or
cooperative processing. At the heart of the
model is the concept of splitting application
functions between a client and a server
processor. The division of labor between the different processors enables the application designer to place an application function on
the processor that is most appropriate for that function. This lets the software designer optimize the use of processors--providing the
greatest possible return on investment for the hardware.
Client/server application design also lets the application provider mask the actual location of application function. The user often does
not know where a specific operation is executing. The entire function may execute in either the PC or server, or the function may be
split between them. This masking of application function locations enables system implementers to upgrade portions of a system over
time with a minimum disruption of application operations, while protecting the investment in existing hardware and software.
TCP/IP Networks:
TCP/IP-based networks play an increasingly important role in computer networks. Perhaps one reason for their appeal is that they are
based on an open specification that is not controlled by any vendor.
What Is TCP/IP?
TCP stands for Transmission Control Protocol and IP stands for Internet Protocol. The term TCP/IP is not limited just to these two
protocols, however. Frequently, the term TCP/IP is used to refer to a group of protocols related to the TCP and IP protocols such as
the User Datagram Protocol (UDP), File Transfer Protocol (FTP), Terminal Emulation Protocol (TELNET), and so on.
The Origins of TCP/IP
In the late 1960s, DARPA (the Defense Advanced Research Project Agency), in the United States, noticed that there was a rapid
proliferation of computers in military communications. Computers, because they can be easily programmed, provide flexibility in
achieving network functions that is not available with other types of communications equipment. The computers then used in military
communications were manufactured by different vendors and were designed to interoperate with computers from that vendor only.
Vendors used proprietary protocols in their communications equipment. The military had a multi vendor network but no common
protocol to support the heterogeneous equipment from different vendors
Net work Cables and Stuff:
In the network you will commonly find three types of cables used these are the, coaxial cable, fiber optic and twisted pair.
Thick Coaxial Cable
This type cable is usually yellow in color and used in what is called thicknets, and has two conductors. This coax can be used in 500meter lengths. The cable itself is made up of a solid center wire with a braided metal shield and plastic sheathing protecting the rest of
the wire.
Thin Coaxial Cable
As with the thick coaxial cable is used in thicknets the thin version is used in thinnets. This type cable is also used called or referred to
as RG-58. The cable is really just a cheaper version of the thick cable.
Fiber Optic Cable
As we all know fiber optics are pretty darn
cool and not cheap. This cable is smaller and
can carry a vast amount of information fast
and over long distances.
Twisted Pair Cables
These come in two flavors of unshielded and
shielded.
Shielded Twisted Pair (STP)
Is more common in high-speed networks. The
biggest difference you will see in the UTP and
STP is that the STP use's metallic shield
wrapping to protect the wire from interference.
-Something else to note about these cables is that they are defined in numbers also. The bigger the number the better the protection
from interference. Most networks should go with no less than a CAT 3 and CAT 5 is most recommended.
-Now you know about cables we need to know about connectors. This is pretty important and you will most likely need the RJ-45
connector. This is the cousin of the phone jack connector and looks real similar with the exception that the RJ-45 is bigger. Most
commonly your connector are in two flavors and this is BNC (Bayonet Naur Connector) used in thicknets and the RJ-45 used in
smaller networks using UTP/STP.
Unshielded Twisted Pair (UTP)
This is the most popular form of cables in the network and the cheapest form that you can go with. The UTP has four pairs of wires
and all inside plastic sheathing. The biggest reason that we call it Twisted Pair is to protect the wires from interference from
themselves. Each wire is only protected with a thin plastic sheath.
Ethernet Cabling
Now to familiarize you with more on the Ethernet and it's cabling we need to look at the 10's. 10Base2, is considered the thin
Ethernet, thinnet, and thinwire which uses light coaxial cable to create a 10 Mbps network. The cable segments in this network can't
be over 185 meters in length. These cables connect with the BNC connector. Also as a note these unused connection must have a
terminator, which will be a 50-ohm terminator.
10Base5, this is considered a thicknet and is used with coaxial cable arrangement such as the BNC connector. The good side to the
coaxial cable is the high-speed transfer and cable segments can be up to 500 meters between nodes/workstations. You will typically
see the same speed as the 10Base2 but larger cable lengths for more versatility.
10BaseT, the “T” stands for twisted as in UTP (Unshielded Twisted Pair) and uses this for 10Mbps of transfer. The down side to this is
you can only have cable lengths of 100 meters between nodes/workstations. The good side to this network is they are easy to set up
and cheap! This is why they are so common an ideal for small offices or homes.
100BaseT, is considered Fast Ethernet uses STP (Shielded Twisted Pair) reaching data transfer of 100Mbps. This system is a little
more expensive but still remains popular as the 10BaseT and cheaper than most other type networks. This on of course would be the
cheap fast version.
10BaseF, this little guy has the advantage of fiber optics and the F stands for just that. This arrangement is a little more complicated
and uses special connectors and NIC's along with hubs to create its network. Pretty darn neat and not to cheap on the wallet.
An important part of designing and installing an Ethernet is selecting the appropriate Ethernet medium. There are four major types of
media in use today: Thickwire for 10BASE5 networks, thin coax for 10BASE2 networks, unshielded twisted pair (UTP) for 10BASE-T
networks and fiber optic for 10BASE-FL or Fiber-Optic Inter-Repeater Link (FOIRL) networks. This wide variety of media reflects the
evolution of Ethernet and also points to the technology's flexibility. Thickwire was one of the first cabling systems used in Ethernet but
was expensive and difficult to use. This evolved to thin coax, which is easier to work with and less expensive.
Networking Basics
Here are some of the fundamental parts of a network:











Network - A network is a group of computers connected together in a way that allows information to be exchanged
between the computers.
Node - A node is anything that is connected to the network. While a node is typically a computer, it can also be
something like a printer or CD-ROM tower.
Segment - A segment is any portion of a network that is separated, by a switch, bridge or router, from other parts of the
network.
Backbone - The backbone is the main cabling of a network that all of the segments connect to. Typically, the backbone
is capable of carrying more information than the individual segments. For example, each segment may have a transfer
rate of 10 Mbps (megabits per second), while the backbone may operate at 100 Mbps.
Topology - Topology is the way that each node is physically connected to the network (more on this in the next section).
Local Area Network (LAN) - A LAN is a network of computers that are in the same general physical location, usually
within a building or a campus. If the computers are far apart (such as across town or in different cities), then a Wide Area
Network (WAN) is typically used.
Network Interface Card (NIC) - Every computer (and most other devices) is connected to a network through an NIC. In
most desktop computers, this is an Ethernet card (normally 10 or 100 Mbps) that is plugged into a slot on the
computer's motherboard.
Media Access Control (MAC) address - This is the physical address of any device -- such as the NIC in a computer -on the network. The MAC address, which is made up of two equal parts, is 6 bytes long. The first 3 bytes identify the
company that made the NIC. The second 3 bytes are the serial number of the NIC itself.
Unicast - A unicast is a transmission from one node addressed specifically to another node.
Multicast - In a multicast, a node sends a packet addressed to a special group address. Devices that are interested in
this group register to receive packets addressed to the group. An example might be a Ciscorouter sending out an update
to all of the other Cisco routers.
Broadcast - In a broadcast, a node sends out a packet that is intended for transmission to all other nodes on the
network.
Network Topologies
Some of the most common topologies in use today include:

Bus - Each node is daisy-chained (connected one right after the other) along the same backbone, similar to Christmas
lights. Information sent from a node travels along the backbone until it reaches its destination node. Each end of a bus
network must be terminated with a resistor to keep the signal that is sent by a node across the network from bouncing
back when it reaches the end of the cable.
Bus network topology

Ring - Like a bus network, rings have the nodes daisy-chained. The difference is that the end of the network comes back
around to the first node, creating a complete circuit. In a ring network, each node takes a turn sending and receiving
information through the use of a token. The token, along with any data, is sent from the first node to the second node,
which extracts the data addressed to it and adds any data it wishes to send. Then, the second node passes the token
and data to the third node, and so on until it comes back around to the first node again. Only the node with the token is
allowed to send data. All other nodes must wait for the token to come to them.
Ring network topology

Star - In a star network, each node is connected to a central device called a hub. The hub takes a signal that comes from
any node and passes it along to all the other nodes in the network. A hub does not perform any type of filtering or routing
of the data. It is simply a junction that joins all the different nodes together.
Star network topology

Star bus - Probably the most common network topology in use today, star bus combines elements of the star and bus
topologies to create a versatile network environment. Nodes in particular areas are connected to hubs (creating stars),
and the hubs are connected together along the network backbone (like a bus network). Quite often, stars are nested
within stars, as seen in the example below:
A typical star bus network
The Problem: Traffic
In the most basic type of network found today, nodes are simply connected together using hubs. As a network grows, there are some
potential problems with this configuration:


Scalability - In a hub network, limited shared bandwidth makes it difficult to accommodate significant growth without
sacrificing performance. Applications today need more bandwidth than ever before. Quite often, the entire network must
be redesigned periodically to accommodate growth.
Latency - This is the amount of time that it takes a packet to get to its destination. Since each node in a hub-based
network has to wait for an opportunity to transmit in order to avoid collisions, the latency can increase significantly as
you add more nodes. Or, if someone is transmitting a large file across the network, then all of the other nodes have to


wait for an opportunity to send their own packets. You have probably seen this before at work -- you try to access a
server or the Internet and suddenly everything slows down to a crawl.
Network failure - In a typical network, one device on a hub can cause problems for other devices attached to the hub
due to incorrect speed settings (100 Mbps on a 10-Mbps hub) or excessive broadcasts. Switches can be configured to
limit broadcast levels.
Collisions - Ethernet uses a process called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to
communicate across the network. Under CSMA/CD, a node will not send out a packet unless the network is clear of
traffic. If two nodes send out packets at the same time, a collision occurs and the packets are lost. Then both nodes wait
a random amount of time and retransmit the packets. Any part of the network where there is a possibility that packets
from two or more nodes will interfere with each other is considered to be part of the same collision domain. A network
with a large number of nodes on the same segment will often have a lot of collisions and therefore a large collision
domain.
While hubs provide an easy way to scale up and shorten the distance that the packets must travel to get from one node to another,
they do not break up the actual network into discrete segments. That is where switches come in. In the next section, you'll find out
how switches assist in directing network traffic.
The Solution: Adding Switches
Imagine that each vehicle is a packet of data waiting for an opportunity to continue on its trip.
Think of a hub as a four-way intersection where everyone has to stop. If more than one car reaches the intersection at the same time,
they have to wait for their turn to proceed.
Now imagine what this would be like with a dozen or even a hundred roads intersecting at a single point. The amount of waiting and
the potential for a collision increases significantly. But wouldn't it be amazing if you could take an exit ramp from any one of those
roads to the road of your choosing? That is exactly what a switch does for network traffic. A switch is like a cloverleaf intersection -each car can take an exit ramp to get to its destination without having to stop and wait for other traffic to go by.
A vital difference between a hub and a switch is that all the nodes connected to a hub share the bandwidth among themselves, while
a device connected to a switch port has the full bandwidth all to itself. For example, if 10 nodes are communicating using a hub on a
10-Mbps network, then each node may only get a portion of the 10 Mbps if other nodes on the hub want to communicate as well. But
with a switch, each node could possibly communicate at the full 10 Mbps. Think about our road analogy. If all of the traffic is coming to
a common intersection, then each car it has to share that intersection with every other car. But a cloverleaf allows all of the traffic to
continue at full speed from one road to the next.
Fully Switched Networks
Image courtesy Cisco Networks
An example of a network using a switch
In a fully switched network, switches replace all the hubs of an Ethernet network with a dedicated segment for every node. These
segments connect to a switch, which supports multiple dedicated segments (sometimes in the hundreds). Since the only devices on
each segment are the switch and the node, the switch picks up every transmission before it reaches another node. The switch then
forwards the frame over the appropriate segment. Since any segment contains only a single node, the frame only reaches the
intended recipient. This allows many conversations to occur simultaneously on a switched network.
Switching allows a network to maintain full-duplex Ethernet. Before switching, Ethernet was half-duplex, which means that data could
be transmitted in only one direction at a time. In a fully switched network, each node communicates only with the switch, not directly
with other nodes. Information can travel from node to switch and from switch to node simultaneously.
Fully switched networks employ either twisted-pair or fiber-optic cabling, both of which use separate conductors for sending and
receiving data. In this type of environment, Ethernet nodes can forgo the collision detection process and transmit at will, since they are
the only potential devices that can access the medium. In other words, traffic flowing in each direction has a lane to itself. This allows
nodes to transmit to the switch as the switch transmits to them -- it's a collision-free environment. Transmitting in both directions can
effectively double the apparent speed of the network when two nodes are exchanging information. If the speed of the network is 10
Mbps, then each node can transmit simultaneously at 10 Mbps.
Mixed Networks
A mixed network with two switches and three hubs
Most networks are not fully switched because of the costs incurred in replacing all of the hubs with switches.
Instead, a combination of switches and hubs are used to create an efficient yet cost-effective network. For example, a company may
have hubs connecting the computers in each department and then a switch connecting all of the department-level hubs.
The OSI Model
The seven layers of the OSI Reference Model
Think of the seven layers as the assembly line in the computer. At each layer, certain things happen to the data that
prepare it for the next layer. The seven layers, which separate into two sets, are:
Application Set

Layer 7: Application - This is the layer that actually interacts with the operating system or application whenever the
user chooses to transfer files, read messages or perform other network-related activities.

Layer 6: Presentation - Layer 6 takes the data provided by the Application layer and converts it into a standard
format that the other layers can understand.

Layer 5: Session - Layer 5 establishes, maintains and ends communication with the receiving device.
Transport Set

Layer 4: Transport - This layer maintains flow control of data and provides for error checking and recovery of data
between the devices. Flow control means that the Transport layer looks to see if data is coming from more than one
application and integrates each application's data into a single stream for the physical network.
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Layer 3: Network - The way that the data will be sent to the recipient device is determined in this layer.
Logical protocols, routing and addressing are handled here.

Layer 2: Data - In this layer, the appropriate physical protocol is assigned to the data. Also, the type of network and
the packet sequencing is defined.

Layer 1: Physical - This is the level of the actual hardware. It defines the physical characteristics of the network
such as connections, voltage levels and timing.
The OSI Reference Model is really just a guideline. Actual protocol stacks often combine one or more of the OSI
layers into a single layer.
Routers and Switches
The OSI Reference Model consists of seven layers that build from the wire (Physical) to the software (Application).
You can see that a switch has the potential to radically change the way nodes communicate with each other. But you may be
wondering what makes it different from a router. Switches usually work at Layer 2 (Data or Datalink) of the OSI Reference Model,
using MAC addresses, while routers work at Layer 3 (Network) with Layer 3 addresses (IP, IPX or Appletalk, depending on
which Layer 3 protocols are being used). The algorithm that switches use to decide how to forward packets is different from the
algorithms used by routers to forward packets.
One of these differences in the algorithms between switches and routers is how broadcasts are handled. On any network, the
concept of a broadcast packet is vital to the operability of a network. Whenever a device needs to send out information but doesn't
know who it should send it to, it sends out a broadcast. For example, every time a new computer or other device comes on to the
network, it sends out a broadcast packet to announce its presence. The other nodes (such as a domain server) can add the computer
to their browser list(kind of like an address directory) and communicate directly with that computer from that point on. Broadcasts are
used any time a device needs to make an announcement to the rest of the network or is unsure of who the recipient of the information
should be.
A hub or a switch will pass along any broadcast packets they receive to all the other segments in the broadcast domain, but a router
will not. Think about our four-way intersection again: All of the traffic passed through the intersection no matter where it was going.
Now imagine that this intersection is at an international border. To pass through the intersection, you must provide the border guard
with the specific address that you are going to. If you don't have a specific destination, then the guard will not let you pass. A router
works like this. Without the specific address of another device, it will not let the data packet through. This is a good thing for keeping
networks separate from each other, but not so good when you want to talk between different parts of the same network. This is where
switches come in.
Network Packet Structure
Most network packets are split into three parts:
Header - The header contains instructions about the data carried by the packet. These instructions may include:

Length of packet (some networks have fixed-length packets, while others rely on the header to contain this
information)

Synchronization (a few bits that help the packet match up to the network)

Packet number (which packet this is in a sequence of packets)

Protocol (on networks that carry multiple types of information, the protocol defines what type of packet is being
transmitted: e-mail, Web page, streaming video)

Destination address (where the packet is going)

Originating address (where the packet came from)
Payload - Also called the body or data of a packet. This is the actual data that the packet is delivering to the
destination. If a packet is fixed-length, then the payload may be padded with blank information to make it the right
size.
Trailer - The trailer, sometimes called the footer, typically contains a couple of bits that tell the receiving device that
it has reached the end of the packet. It may also have some type of error checking. The most common error checking
used in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it works in certain computer
networks: It takes the sum of all the 1s in the payload and adds them together. The result is stored as a hexadecimal
value in the trailer. The receiving device adds up the 1s in the payload and compares the result to the value stored in
the trailer. If the values match, the packet is good. But if the values do not match, the receiving device sends a
request to the originating device to resend the packet.
As an example, let's look at how an e-mail message might get broken into packets. Let's say that you send an e-mail
to a friend. The e-mail is about 3,500 bits (3.5 kilobits) in size. The network you send it over uses fixed-length packets
of 1,024 bits (1 kilobit). The header of each packet is 96 bits long and the trailer is 32 bits long, leaving 896 bits for
the payload. To break the 3,500 bits of message into packets, you will need four packets (divide 3,500 by 896). Three
packets will contain 896 bits of payload and the fourth will have 812 bits. Here is what one of the four packets would
contain:
Each packet's header will contain the proper protocols, the originating address (the IP address of your computer), the
destination address (the IP address of the computer where you are sending the e-mail) and the packet number (1, 2,
3 or 4 since there are 4 packets). Routers in the network will look at the destination address in the header and
compare it to their lookup table to find out where to send the packet. Once the packet arrives at its destination, your
friend's computer will strip the header and trailer off each packet and reassemble the e-mail based on the numbered
sequence of the packets.
Packet-switching
LAN switches rely on packet-switching. The switch establishes a connection between two segments just long enough to send the
current packet. Incoming packets (part of an Ethernet frame) are saved to a temporary memory area (buffer); the MAC address
contained in the frame's header is read and then compared to a list of addresses maintained in the switch's lookup table. In an
Ethernet-based LAN, an Ethernet frame contains a normal packet as the payload of the frame, with a special header that includes the
MAC address information for the source and destination of the packet.
Packet-based switches use one of three methods for routing traffic:
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Cut-through
Store-and-forward
Fragment-free
Cut-through switches read the MAC address as soon as a packet is detected by the switch. After storing the 6 bytes that make up
the address information, they immediately begin sending the packet to the destination node, even as the rest of the packet is coming
into the switch.
A switch using store-and-forward will save the entire packet to the buffer and check it for CRC errors or other problems before
sending. If the packet has an error, it is discarded. Otherwise, the switch looks up the MAC address and sends the packet on to the
destination node. Many switches combine the two methods, using cut-through until a certain error level is reached and then changing
over to store-and-forward. Very few switches are strictly cut-through, since this provides no error correction.
A less common method is fragment-free. It works like cut-through except that it stores the first 64 bytes of the packet before sending
it on. The reason for this is that most errors, and all collisions, occur during the initial 64 bytes of a packet.
Local Area vs. Wide Area
We can classify network technologies as belonging to one of two basic groups. Local area network(LAN)
technologies connect many devices that are relatively close to each other, usually in the same building. The library
terminals that display book information would connect over a local area network. Wide area network (WAN)
technologies connect a smaller number of devices that can be many kilometers apart. For example, if two libraries at
the opposite ends of a city wanted to share their book catalog information, they would most likely make use of a wide
area network technology, which could be a dedicated line leased from the local telephone company, intended solely
to carry their data.
In comparison to WANs, LANs are faster and more reliable, but improvements in technology continue to blur the line
of demarcation. Fiber optic cables have allowed LAN technologies to connect devices tens of kilometers apart, while
at the same time greatly improving the speed and reliability of WANs.
The Ethernet
In 1973, at Xerox Corporation’s Palo Alto Research Center (more commonly known as PARC), researcher Bob
Metcalfe designed and tested the first Ethernet network. While working on a way to link Xerox’s "Alto" computer to
a printer, Metcalfe developed the physical method of cabling that connected devices on the Ethernet as well as the
standards that governed communication on the cable. Ethernet has since become the most popular and most widely
deployed network technology in the world. Many of the issues involved with Ethernet are common to many network
technologies, and understanding how Ethernet addressed these issues can provide a foundation that will improve
your understanding of networking in general.
The Ethernet standard has grown to encompass new technologies as computer networking has matured, but the
mechanics of operation for every Ethernet network today stem from Metcalfe’s original design. The original Ethernet
described communication over a single cable shared by all devices on the network. Once a device attached to this
cable, it had the ability to communicate with any other attached device. This allows the network to expand to
accommodate new devices without requiring any modification to those devices already on the network.
Ethernet Basics
Ethernet is a local area technology, with networks traditionally operating within a single building, connecting devices
in close proximity. At most, Ethernet devices could have only a few hundred meters of cable between them, making
it impractical to connect geographically dispersed locations. Modern advancements have increased these distances
considerably, allowing Ethernet networks to span tens of kilometers.
Protocols
In networking, the term protocol refers to a set of rules that govern communications. Protocols are to computers
what language is to humans. Since this article is in English, to understand it you must be able to read English.
Similarly, for two devices on a network to successfully communicate, they must both understand the same protocols.
Ethernet Terminology
Ethernet follows a simple set of rules that govern its basic operation. To better understand these rules, it is important
to understand the basics of Ethernet terminology.
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Medium - Ethernet devices attach to a common medium that provides a path along which the electronic signals will
travel. Historically, this medium has been coaxial copper cable, but today it is more commonly a twisted pair or fiber
optic cabling.
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Segment - We refer to a single shared medium as an Ethernet segment.
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Node - Devices that attach to that segment are stations or nodes.
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Frame - The nodes communicate in short messages called frames, which are variably sized chunks of information.
Frames are analogous to sentences in human language. In English, we have rules for constructing our sentences:
We know that each sentence must contain a subject and a predicate. The Ethernet protocolspecifies a set of rules
for constructing frames. There are explicit minimum and maximum lengths for frames, and a set of required pieces of
information that must appear in the frame. Each frame must include, for example, both a destination address and
a source address, which identify the recipient and the sender of the message. The address uniquely identifies the
node, just as a name identifies a particular person. No two Ethernet devices should ever have the same address.
Ethernet Medium
Since a signal on the Ethernet medium reaches every attached node, the destination address is critical to identify the
intended recipient of the frame.
For example, in the figure above, when computer B transmits to printer C, computers A and D will still receive and
examine the frame. However, when a station first receives a frame, it checks the destination address to see if the
frame is intended for itself. If it is not, the station discards the frame without even examining its contents.
One interesting thing about Ethernet addressing is the implementation of a broadcast address. A frame with a
destination address equal to the broadcast address (simply called a broadcast, for short) is intended for every node
on the network, and every node will both receive and process this type of frame.
CSMA/CD
The acronym CSMA/CD signifies carrier-sense multiple access with collision detection and describes how the
Ethernet protocol regulates communication among nodes. While the term may seem intimidating, if we break it apart
into its component concepts we will see that it describes rules very similar to those that people use in polite
conversation. To help illustrate the operation of Ethernet, we will use an analogy of a dinner table conversation.
Let’s represent our Ethernet segment as a dinner table, and let several people engaged in polite conversation at the
table represent the nodes. The term multiple access covers what we already discussed above: When one Ethernet
station transmits, all the stations on the medium hear the transmission, just as when one person at the table talks,
everyone present is able to hear him or her.
Now let's imagine that you are at the table and you have something you would like to say. At the moment, however, I
am talking. Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I
finished talking before making your statement. This is the same concept described in the Ethernet protocol as carrier
sense. Before a station transmits, it "listens" to the medium to determine if another station is transmitting. If the
medium is quiet, the station recognizes that this is an appropriate time to transmit.
Collision Detection
Carrier-sense multiple access gives us a good start in regulating our conversation, but there is one scenario we still
need to address. Let’s go back to our dinner table analogy and imagine that there is a momentary lull in the
conversation. You and I both have something we would like to add, and we both "sense the carrier" based on the
silence, so we begin speaking at approximately the same time. In Ethernet terminology, a collision occurs when we
both spoke at once.
In our conversation, we can handle this situation gracefully. We both hear the other speak at the same time we are
speaking, so we can stop to give the other person a chance to go on. Ethernet nodes also listen to the medium while
they transmit to ensure that they are the only station transmitting at that time. If the stations hear their own
transmission returning in a garbled form, as would happen if some other station had begun to transmit its own
message at the same time, then they know that a collision occurred. A single Ethernet segment is sometimes called
a collision domain because no two stations on the segment can transmit at the same time without causing a
collision. When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to
transmit when they again detect silence on the medium.
The random pause and retry is an important part of the protocol. If two stations collide when transmitting once, then
both will need to transmit again. At the next appropriate chance to transmit, both stations involved with the previous
collision will have data ready to transmit. If they transmitted again at the first opportunity, they would most likely
collide again and again indefinitely. Instead, the random delay makes it unlikely that any two stations will collide more
than a few times in a row.
Limitations of Ethernet
A single shared cable can serve as the basis for a complete Ethernet network, which is what we discussed above.
However, there are practical limits to the size of our Ethernet network in this case. A primary concern is the length of
the shared cable.
Electrical signals propagate along a cable very quickly, but they weaken as they travel, and electrical interference
from neighboring devices (fluorescent lights, for example) can scramble the signal. A network cable must be short
enough that devices at opposite ends can receive each other's signals clearly and with minimal delay. This places a
distance limitation on the maximum separation between two devices (called the network diameter) on an Ethernet
network. Additionally, since in CSMA/CD only a single device can transmit at a given time, there are practical limits to
the number of devices that can coexist in a single network. Attach too many devices to one shared segment and
contention for the medium will increase. Every device may have to wait an inordinately long time before getting a
chance to transmit.
Engineers have developed a number of network devices that alleviate these difficulties. Many of these devices are
not specific to Ethernet, but play roles in other network technologies as well.
Repeaters
The first popular Ethernet medium was a copper coaxial cable known as "thicknet." The maximum length of a thicknet
cable was 500 meters. In large building or campus environments, a 500-meter cable could not always reach every
network device. A repeater addresses this problem.
Repeaters connect multiple Ethernet segments, listening to each segment and repeating the signal heard on one
segment onto every other segment connected to the repeater. By running multiple cables and joining them with
repeaters, you can significantly increase your network diameter.
Segmentation
In our dinner table analogy, we had only a few people at a table carrying out the conversation, so restricting
ourselves to a single speaker at any given time was not a significant barrier to communication. But what if there were
many people at the table and only one were allowed to speak at any given time?
In practice, we know that the analogy breaks down in circumstances such as these. With larger groups of people, it is
common for several different conversations to occur simultaneously. If only one person in a crowded room or at a
banquet dinner were able to speak at any time, many people would get frustrated waiting for a chance to talk. For
humans, the problem is self-correcting: Voices only carry so far, and the ear is adept at picking out a particular
conversation from the surrounding noise. This makes it easy for us to have many small groups at a party converse in
the same room; but network cables carry signals quickly and efficiently over long distances, so this natural
segregation of conversations does not occur.
Ethernet networks faced congestion problems as they increased in size. If a large number of stations connected to
the same segment and each generated a sizable amount of traffic, many stations may attempt to transmit whenever
there was an opportunity. Under these circumstances, collisions would become more frequent and could begin to
choke out successful transmissions, which could take inordinately large amounts of time to complete. One way to
reduce congestion would be to split a single segment into multiple segments, thus creating multiple collision
domains. This solution creates a different problem, as now these now separate segments are not able to share
information with each other.
Bridges
To alleviate problems with segmentation, Ethernet networks implemented bridges. Bridges connect two or more
network segments, increasing the network diameter as a repeater does, but bridges also help regulate traffic. They
can send and receive transmissions just like any other node, but they do not function the same as a normal node.
The bridge does not originate any traffic of its own; like a repeater, it only echoes what it hears from other stations.
(That last statement is not entirely accurate: Bridges do create a special Ethernet frame that allows them to
communicate with other bridges, but that is outside the scope of this article.)
Remember how the multiple access and shared medium of Ethernet meant that every station on the wire received
every transmission, whether it was the intended recipient or not? Bridges make use of this feature to relay traffic
between segments. In the figure above, the bridge connects segments 1 and 2. If station A or B were to transmit, the
bridge would also receive the transmission on segment 1. How should the bridge respond to this traffic? It could
automatically transmit the frame onto segment 2, like a repeater, but that would not relieve congestion, as the
network would behave like one long segment.
One goal of the bridge is to reduce unnecessary traffic on both segments. It does this by examining the destination
address of the frame before deciding how to handle it. If the destination address is that of station A or B, then there is
no need for the frame to appear on segment 2. In this case, the bridge does nothing. We can say that the
bridge filters or drops the frame. If the destination address is that of station C or D, or if it is the broadcast address,
then the bridge will transmit, or forward the frame on to segment 2. By forwarding packets, the bridge allows any of
the four devices in the figure to communicate. Additionally, by filtering packets when appropriate, the bridge makes it
possible for station A to transmit to station B at the same time that station C transmits to station D, allowing two
conversations to occur simultaneously!
Switches are the modern counterparts of bridges, functionally equivalent but offering a dedicated segment for every
node on the network (more on switches later in the article).
Routers: Logical Segmentation
Bridges can reduce congestion by allowing multiple conversations to occur on different segments simultaneously, but
they have their limits in segmenting traffic as well.
An important characteristic of bridges is that they forward Ethernet broadcasts to all connected segments. This
behavior is necessary, as Ethernet broadcasts are destined for every node on the network, but it can pose problems
for bridged networks that grow too large. When a large number of stations broadcast on a bridged network,
congestion can be as bad as if all those devices were on a single segment.
Routers are advanced networking components that can divide a single network into two logically separate networks.
While Ethernet broadcasts cross bridges in their search to find every node on the network, they do not cross routers,
because the router forms a logical boundary for the network.
Routers operate based on protocols that are independent of the specific networking technology, like Ethernet or
token ring (we'll discuss token ring later). This allows routers to easily interconnect various network technologies,
both local and wide area, and has led to their widespread deployment in connecting devices around the world as part
of the global Internet.
Switched Ethernet
Modern Ethernet implementations often look nothing like their historical counterparts. Where long runs of coaxial
cable provided attachments for multiple stations in legacy Ethernet, modern Ethernet networks use twisted pair wiring
or fiber optics to connect stations in a radial pattern. Where legacy Ethernet networks transmitted data at
10 megabits per second (Mbps), modern networks can operate at 100 or even 1,000 Mbps!
Perhaps the most striking advancement in contemporary Ethernet networks is the use of switched Ethernet.
Switched networks replace the shared medium of legacy Ethernet with a dedicated segment for each station. These
segments connect to a switch, which acts much like an Ethernet bridge, but can connect many of these single station
segments. Some switches today can support hundreds of dedicated segments. Since the only devices on the
segments are the switch and the end station, the switch picks up every transmission before it reaches another node.
The switch then forwards the frame over the appropriate segment, just like a bridge, but since any segment contains
only a single node, the frame only reaches the intended recipient. This allows many conversations to occur
simultaneously on a switched network. (See How LAN Switches work to learn more about switching technology.)
Full-duplex Ethernet
Ethernet switching gave rise to another advancement, full-duplex Ethernet. Full-duplex is a data communications
term that refers to the ability to send and receive data at the same time.
Legacy Ethernet is half-duplex, meaning information can move in only one direction at a time. In a totally switched
network, nodes only communicate with the switch and never directly with each other. Switched networks also employ
either twisted pair or fiber optic cabling, both of which use separate conductors for sending and receiving data. In this
type of environment, Ethernet stations can forgo the collision detection process and transmit at will, since they are
the only potential devices that can access the medium. This allows end stations to transmit to the switch at the same
time that the switch transmits to them, achieving acollision-free environment.
IP Addressing:
An IP (Internet Protocol) address is a unique identifier for a node or host connection on an IP network. An IP address is a 32 bit binary
number usually represented as 4 decimal values, each representing 8 bits, in the range 0 to 255 (known as octets) separated by
decimal points. This is known as "dotted decimal" notation.
Example: 140.179.220.200
It is sometimes useful to view the values in their binary form.
140 .179 .220 .200
10001100.10110011.11011100.11001000
Every IP address consists of two parts, one identifying the network and one identifying the node. The Class of the address and the
subnet mask determine which part belongs to the network address and which part belongs to the node address.
Address Classes:
There are 5 different address classes. You can determine which class any IP address is in by examining the first 4 bits of the IP
address.
Class A addresses begin with 0xxx, or 1 to 126 decimal.
Class B addresses begin with 10xx, or 128 to 191 decimal.
Class C addresses begin with 110x, or 192 to 223 decimal.
Class D addresses begin with 1110, or 224 to 239 decimal.
Class E addresses begin with 1111, or 240 to 254 decimal.
Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and for internal testing on a local machine. [You can
test this: you should always be able to ping 127.0.0.1, which points to yourself] Class D addresses are reserved for multicasting. Class
E addresses are reserved for future use. They should not be used for host addresses.
Now we can see how the Class determines, by default, which part of the IP address belongs to the network (N) and which part
belongs to the node (n).
Class A -- NNNNNNNN.nnnnnnnn.nnnnnnn.nnnnnnn
Class B -- NNNNNNNN.NNNNNNNN.nnnnnnnn.nnnnnnnn
Class C -- NNNNNNNN.NNNNNNNN.NNNNNNNN.nnnnnnnn
In the example, 140.179.220.200 is a Class B address so by default the Network part of the address (also known as the Network
Address) is defined by the first two octets (140.179.x.x) and the node part is defined by the last 2 octets (x.x.220.200).
In order to specify the network address for a given IP address, the node section is set to all "0"s. In our example, 140.179.0.0
specifies the network address for 140.179.220.200. When the node section is set to all "1"s, it specifies a broadcast that is sent to all
hosts on the network. 140.179.255.255 specifies the example broadcast address. Note that this is true regardless of the length of the
node section.
Private Subnets:
There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0/8, 172.16.0.0/12, and
192.168.0.0/16. They can be used by anyone setting up internal IP networks, such as a lab or home LAN behind a NAT or proxy
server or a router. It is always safe to use these because routers on the Internet will never forward packets coming from these
addresses
Subnetting an IP Network can be done for a variety of reasons, including organization, use of different physical media (such as
Ethernet, FDDI, WAN, etc.), preservation of address space, and security. The most common reason is to control network traffic. In an
Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. Performance can be
adversely affected under heavy traffic loads, due to collisions and the resulting retransmissions. A router is used to connect IP
networks to minimize the amount of traffic each segment must receive.
Subnet Masking
Applying a subnet mask to an IP address allows you to identify the network and node parts of the address. The network bits are
represented by the 1s in the mask, and the node bits are represented by the 0s. Performing a bitwise logical AND operation between
the IP address and the subnet mask results in the Network Address or Number.
For example, using our test IP address and the default Class B subnet mask, we get:
10001100.10110011.11110000.11001000 140.179.240.200 Class B IP Address
11111111.11111111.00000000.00000000 255.255.000.000 Default Class B Subnet Mask
10001100.10110011.00000000.00000000 140.179.000.000 Network Address
Default subnet masks:
Class A - 255.0.0.0 - 11111111.00000000.00000000.00000000
Class B - 255.255.0.0 - 11111111.11111111.00000000.00000000
Class C - 255.255.255.0 - 11111111.11111111.11111111.00000000
CIDR -- Classless InterDomain Routing.
CIDR was invented several years ago to keep the internet from running out of IP addresses. The "classful" system of allocating IP
addresses can be very wasteful; anyone who could reasonably show a need for more that 254 host addresses was given a Class B
address block of 65533 host addresses. Even more wasteful were companies and organizations that were allocated Class A address
blocks, which contain over 16 Million host addresses! Only a tiny percentage of the allocated Class A and Class B address space has
ever been actually assigned to a host computer on the Internet.
People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of
address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992
as a scheme called Supernetting.
The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation
(Class A = /8, Class B = /16, and Class C = /24)
It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get
them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 5 years ago, there were less than
5000 network routes in the entire Internet. Today, there are over 90,000. Using CIDR, the biggest ISPs are allocated large chunks of
address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated
networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network
route on the Internet.
It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit
addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for
every person on earth
Basic Guide to DNS
DNS
DNS stands for Domain Name System. This system is in place to organize and identify domains. Essentially, DNS
provides a name for a domain's one or more IP addresses. For instance, the domain name wolf.example.com might
translate to198.102.434.8. This makes it much easier to remember URLs and email addresses.
DNS is also used to find out where to deliver email for a particular address. This is done with MX Records.
You need to have a registered domain name to use Google Apps.
Domain Name
Domain names are easy-to-remember names (URLs and email addresses) that are associated with one or more IP
addresses. Since a web page is defined by its URL, the page can move to a different IP address without affecting
visitors.
Example: www.singlespeed.com




singlespeed.com is the domain name.
com is the top level domain.
singlespeed is a subdivision of com, and represents the second-level domain.
www is a subdomain (also known as third-level domain or CNAME).
The whole domain name can not exceed a total length of 255 characters, but some registries have shorter limits.
Domain Registrar
Domain registrars sell Internet domain names (ex. blueshirt.com or organicfood.org ). Most of these companies offer
ahosting service in addition to registration.
If your domain registrar is separate from your domain host, you'll need to add the host's name servers to your
registrar's account. For example, if you purchase a domain name from namecheap.com (which offers domain
registration) and host your domain with DynDNS (which offers domain hosting), you'll add the name servers of
DynDNS (ns1.mydyndns.org andns2.mydyndns.org) to your account with namecheap.com.
Google Apps offers domain registration with a select group of domain registration partners. This allows you to
purchase a domain name and sign up for Google Apps at the same time. If you purchase a domain name while
signing up, Google will auto-configure services for your domain so that you won't need to manually configure MX and
CNAME records.
If you purchased your domain name before signing up for Google Apps, visit our list of domain hosts (some of which
are also domain registrars) that have instructions for modifying MX records in our Help Center.
Top Level Domain
Top-level domains are the last part of a domain name - the letters after the last period. Some examples are: biz com
org edu us ca fr de travel local es pl
Second-level Domain
Second-level domains are directly below top-level domains. Some current examples are:
Second-level Domain Domain Name
Google
google.com
Wikipedia
wikipedia.org
Ontariotravel
ontariotravel.com
Craigslist
craigslist.com
louvre
louvre.fr
Third-level Domain
Third-level domains are also known as subdomains and CNAMEs. In a URL, the subdomain is written before the
domain name. Here's some examples:
Subdomain URL
affiliates
http://affiliates.art.com
www
http://www.rockfound.org
men
http://men.style.com
mail
http://mail.google.com
bus
http://www.bus.umich.edu
To set up web publishing with Google Apps, you'll need to pick a subdomain as your web publishing address.
Domain Host
Domain hosts run DNS servers for your domain. This includes A records, MX records, and CNAME records. Most
domain hosts offer domain name registration as well.
Since Google Apps is not a domain host, you'll need to modify your DNS records with your domain host to set up
email and web publishing. Click here if you don't know which company is hosting your domain.
A Record
A records (also known as host records) are the central records of DNS. These records link a domain, or subdomain,
to an IP address.
A records and IP addresses do not necessarily match on a one-to-one basis. Many A records correspond to a single
IP address, where one machine can serve many web sites. Alternatively, a single A record may correspond to many
IP addresses. This can facilitate fault tolerance and load distribution, and allows a site to move its physical location.
Google Apps does not support IP addresses alone. Instead of using A records, you can set up email and web
publishing by modifying your MX and CNAME records with your domain host.
NS Record
Name server records determine which servers will communicate DNS information for a domain. Two NS records
must be defined for each domain. Generally, you will have a primary and a secondary name server record - NS
records are updated with your domain registrar and will take 24-72 hours to take effect.
If your domain registrar is separate from your domain host, your host will provide two name servers that you can use
to update your NS records with your registrar.
If you're not sure who is hosting your domain, you can perform a free NS Lookup. Here's how:
1.
2.
Visit Google.com.
Search for NS lookup.
3.
4.
5.
6.
Select a search result.
Type your domain name into the tool.
Select NS records or Any records for your query.
Click Look it up.
Example result (showing that name-services.com is the domain host for mightydinosaur.com):
mightydinosaur.com nameserver = dns1.name-services.com.
MX Record
Mail Exchange records direct email to servers for a domain, and are listed in order of priority. If mail can't be
delivered using the first priority record, the second priority record is used, and so on.
To set up email with Google Apps, you need to configure the MX records with your domain host using Google's
server information.
If you'd like to check the status of your MX records, go to the Email settings page (Settings > Email in the Google
Apps administrator control panel. Your MX records appear in the MX Records section of the page.
CNAME Record
Canonical name records are aliases for A records. For each CNAME record, you can choose an alias and a host.
To set up web publishing with Google Apps, you can pick an address for your web pages. The third-level domain of
the address is the alias and ghs.google.com is the host.
If you'd like to check the status of your CNAME record for web publishing, you can perform a free CNAME lookup.
Here's how:
1.
2.
Visit Google.com.
Search for NS lookup.
3.
4.
5.
6.
Select a search result from the list.
Type your web publishing address in to the field.
Select CNAME record if it's not the default search query.
Click Submit, or Lookup.
Example result (showing that the subdomain of start.mightydinosaur.com is pointing to ghs.google.com):
DNS Lookup (CNAME) for start.mightydinosaur.com. Items Returned: 1
 ghs.google.com
IP Address
Internet Protocol addresses are unique numbers that allow devices to locate information on a network.
Since a domain name may have one or more associated IP addresses, Google Apps doesn't support email and web
publishing configuration using IP addresses alone.
Custom URLs
Custom URLs, or short URLs, make using the Internet easier. A custom URL allows you and your users to access
the login page for services at your domain with a simple, easy-to-remember address. With Google Apps, your custom
URLs will follow this format:
http://[customize this section].your_domain.com
Instead of asking your users to visit http://www.google.com/calendar/a/your_domain.com to log in to their calendars,
you can create a short, custom URL. Learn more
Calendar Examples
http://calendar.your_domain.com
http://c.your_domain.com
http://9-5.your_domain.com
http://myagenda.your_domain.com
http://where2go.your_domain.com
Domain Alias
Domain name aliases are additional domain names associated with your primary domain. With Google Apps, you
can add a domain alias that receives mail and delivers it to mailboxes at your primary domain.
Some common uses:



Add a domain alias to cover other top-level domains. If your domain name is theurbanexperience.org,
you may want to alias theurbanexperience.com and theurbanexperience.us.
Add a domain alias to help people who mistype your domain name. If your domain name
is theurbanexperience.org, you may want to alias urbanexperience.org, theurbanexperiment.org,
and urbanexperiences.org.
Add a domain alias to receive mail addressed to two separate domains in one mailbox. If you receive mail
at two domain names, such as admin@bradford.com and admin@clarkston.com, you can
alias clarkston.com tobradford.com, and all mail sent to either address will be delivered
to admin@bradford.com.
WHOIS directory
The WHOIS directory is a public listing of domain names, and people or organizations associated with each domain
name.
As a privacy measure, some domain name owners prefer to have their personal information hidden from the WHOIS
directory. This is similar to the way someone may want his/her personal telephone number unlisted in a local
telephone book.
The WHOIS directory is used to determine the owner of domain names and IP addresses. There are many free webbased directories available on the Internet. The information provided in the WHOIS directory includes a mailing
address and a telephone number.
Examining your network with commands:
Ping
PING is used to check for a response from another computer on the network. It can tell you a great deal of information about the
status of the network and the computers you are communicating with.
Ping returns different responses depending on the computer in question. The responses are similar depending on the options used.
Ping uses IP to request a response from the host. It does not use TCP
.It takes its name from a submarine sonar search - you send a short sound burst and listen for an echo - a ping - coming back.
In an IP network, `ping' sends a short data burst - a single packet - and listens for a single packet in reply. Since this tests the most
basic function of an IP network (delivery of single packet), it's easy to see how you can learn a lot from some `pings'.
To stop ping, type control-c. This terminates the program and prints out a nice summary of the number of packets transmitted, the
number received, and the percentage of packets lost, plus the minimum, average, and maximum round-trip times of the packets.
Sample ping session
PING localhost (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=4 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=5 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=6 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=7 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=8 ttl=255 time=2 ms
64 bytes from 127.0.0.1: icmp_seq=9 ttl=255 time=2 ms
localhost ping statistics
10 packets transmitted, 10 packets received, 0% packet loss
round-trip min/avg/max = 2/2/2 ms
meikro$
The Time To Live (TTL) field can be interesting. The main purpose of this is so that a packet doesn't live forever on the network and
will eventually die when it is deemed "lost." But for us, it provides additional information. We can use the TTL to determine
approximately how many router hops the packet has gone through. In this case it's 255 minus N hops, where N is the TTL of the
returning Echo Replies. If the TTL field varies in successive pings, it could indicate that the successive reply packets are going via
different routes, which isn't a great thing.
The time field is an indication of the round-trip time to get a packet to the remote host. The reply is measured in milliseconds. In
general, it's best if round-trip times are under 200 milliseconds. The time it takes a packet to reach its destination is called latency. If
you see a large variance in the round-trip times (which is called "jitter"), you are going to see poor performance talking to the host
NSLOOKUP
NSLOOKUP is an application that facilitates looking up hostnames on the network. It can reveal the IP address of a host or, using the
IP address, return the host name.
It is very important when troubleshooting problems on a network that you can verify the components of the networking process.
Nslookup allows this by revealing details within the infrastructure.
NETSTAT
NETSTAT is used to look up the various active connections within a computer. It is helpful to understand what computers or networks
you are connected to. This allows you to further investigate problems. One host may be responding well but another may be less
responsive.
IPconfig
This is a Microsoft windows NT, 2000 command. It is very useful in determining what could be wrong with a network.
This command when used with the /all switch, reveal enormous amounts of troubleshooting information within the system.
Windows 2000 IP Configuration
Host Name . . . . . . . . . . . . : cowder
Primary DNS Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Broadcast
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . :
WAN (PPP/SLIP) Interface
Physical Address. . . . . . . . . :
00-53-45-00-00-00
DHCP Enabled. . . . . . . . . . . : No
IP Address. . . . . . . . . . . . : 12.90.108.123
Subnet Mask . . . . . . . . . . . : 255.255.255.255
Default Gateway . . . . . . . . . : 12.90.108.125
DNS Servers . . . . . . . . . . . : 12.102.244.2
204.127.129.2
Traceroute
Traceroute on Unix and Linux (or tracert in the Microsoft world) attempts to trace the current network path to a destination. Here is an
example of a traceroute run to www.berkeley.edu:
$ traceroute www.berkeley.edu
traceroute to amber.Berkeley.EDU (128.32.25.12), 30 hops max, 40 byte packets
1 sf1-e3.wired.net (206.221.193.1) 3.135 ms 3.021 ms 3.616 ms
2 sf0-e2s2.wired.net (205.227.206.33) 1.829 ms 3.886 ms 2.772 ms
3 paloalto-cr10.bbnplanet.net (131.119.26.105) 5.327 ms 4.597 ms 5.729 ms
4 paloalto-br1.bbnplanet.net (131.119.0.193) 4.842 ms 4.615 ms 3.425 ms
5 sl-sj-2.sprintlink.net (4.0.1.66) 7.488 ms 38.804 ms 7.708 ms
6 144.232.8.81 (144.232.8.81) 6.560 ms 6.631 ms 6.565 ms
7 144.232.4.97 (144.232.4.97) 7.638 ms 7.948 ms 8.129 ms
8 144.228.146.50 (144.228.146.50) 9.504 ms 12.684 ms 16.648 ms
9 f5-0.inr-666-eva.berkeley.edu (198.128.16.21) 9.762 ms 10.611 ms 10.403 ms
10 f0-0.inr-107-eva.Berkeley.EDU (128.32.2.1) 11.478 ms 10.868 ms 9.367 ms
11 f8-0.inr-100-eva.Berkeley.EDU (128.32.235.100) 10.738 ms 11.693 ms 12.520 ms