Computer Networks:
Computer Network consists of two or more computers connected to each
other so that they can exchange information. Broadly, Networks are divided into
three categories: LAN, WAN & MAN.
Local Area Network (LAN) is collection of computers and other connected devices
that fit within the scope of a single physical network. LANs provide the building
blocks for other big networks. A WAN can span across the world. 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, satellite links etc. MANs use WAN
technologies to interconnect LANs within a specific geographical region, such as a
city.
Network Building Blocks:
Client Computers: The computers used by END USERS to access the Network
resources. These computers are loaded with “Client” operating systems such as
Windows98, WindowsXP or Client installation of Linux etc. These are also called
Workstations.
Server Computers: These computers provide resources – shared storage, shared
printers, Network services, E-Mail, Internet services , Data servers etc. These
computers are hardware wise powerful. Network Operating systems are loaded on
these computers such as Windows Server 2000, 2003, Linux server, Unix server etc.
Peer to Peer Networking: All the computers in a network can share resources with
each other. Any computer, for example can share a printer, Hard drive with other
computers. All the computers are also used by individual users for day to day work.
No computer is “special”. Peer-to-peer networks have no centralized control over
shared resources. Each network user controls access to the resources that reside on
his or her machine. As networks grow, the decentralized nature of this type of
network makes locating resources increasingly difficult, as the number of peers to be
checked goes up. Peer-to-peer networks are the easiest and cheapest kind of
network to install. These are used for small Networks. Peer-to-peer networks have
no centralized security or maintenance functions.
Here are some benefits of peer-to-peer networks:
 They are easy to install and configure.
 Users are able to control their own shared resources.
 This type of network is inexpensive to purchase and operate.
 You don’t need any equipment or software other than an
operating system.
 Users might have to remember as many passwords as there
are shared resources.
 You must perform individual backups on each machine to
Protect all shared data.
Peer to peer networking features are built into Windows Operating Systems. No
additional software is required to establish this type of networking.
Dedicated Server or Client-Server networking: In this setup, Server computer is
the server only. It provides shared resources & services. Data base Servers, Intranet
Servers are usually dedicated servers. Server-based networks also provide
centralized verification of user accounts and passwords. Also, only network
administrators can modify the security information in the domain controller’s
database. This approach provides centralized security. Network resources like files
and printers are easier to find because they are generally located on specific servers,
not on individual user machines across the network. Dedicated Server type network
suffers from single point of failure on networks.
Network Topology:
This term refers to the way or shape in which computers are connected in a network.
Star: All the nodes are connected to a centralized hub. Maintenance is easy, but
signal gets divided. UTP cable length is 100meters.
BUS: All the devices are connected to a common bus or Backbone. This type of set
up is difficult for troubleshooting.
MESH: Every device is connected to every other device. Connections are redundant.
These types are reliable.
RING: Devices are wired in a circle. This type uses “token ring” type of
communication. “Collisions” of packets is avoided, but fault finding is difficult.
Open System Interconnection (OSI) model :
The OSI model breaks the various aspects of Networking functions into seven layers.
It is a framework in which various real networking standards are expected to fit.
Application Layer ( No.7) : This layer enables Applications to use the Network. It
manages data flow control & handles errors. It is nearest to user interface.
Presentation Layer (No.6) : Data is translated in this layer into a form suitable for
Application layer. Data encryption, compression is also handled here.
Session Layer (No. 5) : This layer facilitates the connecting Applications to
establish a session. After a session between two devices is established, it is
maintained till it is terminated by either device. Transmission modes such as half
duplex, full duplex are decided in this layer.
Transport Layer (No.4) : Handles transportation between devices. Manages packet
handling, i.e. large messages are broken into packets for efficient handling & ensures
error free delivery. Network protocols such as TCP &SPX work at this layer.
Network Layer(No.3): Actual routing of the messages in the network is managed
here. It manages translation of names into addresses, network traffic problems,
routing etc. Network protocols such as IP & IPX work on this layer.
Data Link Layer (No.2) : In this layer, data is sent from the network layer to
physical layer. It handles physical communication between two systems in the form
of packets or frames. MAC addresses exist at this layer. Data transmission from
many nodes without collision is handled at this layer.
Physical : ( Layer 1) : Transmits data over a physical medium. Defines physical
aspects such as cards, cables etc. Data is sent as bits.
The Ethernet Protocol :
Ethernet protocol operates at the first two layers identified in the OSI model – the
Physical layer & the data link layer. IEEE Standard 802.3 standard defines rules for
configuring an Ethernet network as well as specifying how elements in an Ethernet
network interact with one another. 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 FiberOptic Inter-Repeater Link (FOIRL) networks. The most popular wiring schemes are
10BASE-T and 100BASE-TX, which use unshielded twisted pair (UTP) cable.
10 Base5: This means 10 Mb/s transmission rate, the baseband form of
transmission, and the 500 meter maximum supported segment length. This is known
as “thick” Ethernet, which uses a co-axial cable.
10 Base2: This uses a thinner co-axial cable, just like a TV cable. It uses the BUS
topology.
10 BaseT: This is the most popular version of Ethernet. These networks use the
Star topology. Maximum length is 100 meters. There are four pairs of twisted pairs
of cables. 10 baseT uses only two of these pairs.
10 BaseFL: This was the first standard for Fiber Optic cable.
FAST ETHERNET:
This refers to Ethernet that runs at 100Mbps:
100BaseT4: Over the same UTP cable(10BaseT), 100Mbps is supported. All the foir
pairs of bable are used.
100BaseTX: The most commonly used standard today for Office Networking at
100Mbps. AS Category 5 cable needs to be used for 100Mbps. Only two pairs of
wires are used.
100BaseFX: This is the Fiber Optic version of 100Mbps Ethernet. Fiber Optic cable is
commonly used as a Network Backbone.
Cable:
Twisted Pair Ethernet (10baseT), sometime also called "UTP" (Unshielded
Twisted Pair) is based on using a cable similar to phone-wiring. The cable is
connected via an RJ-45 connectors to the network card installed in the PC.
 The Eight-conductor cable contains 4 pairs of wires. Each pair consists of a
colored wire and a white wire with a stripe of the same color. The pairs are
twisted together. To maintain reliability on Ethernet, you should not untwist
them.
 The pairs designated for 10BaseT Ethernet are Orange and Green. The other two
pairs, Brown and Blue, can be used for a second Ethernet line or for phone
connections.
There are two basic cables connections. A straight through cable connection scheme,
which is used to connect to a hub or switch, and a cross-over cable connection
scheme for connecting two computers without using a HUB.
 The cable is connected to an RJ-45 connector.
RJ-45 Plug : It is a big size telephone type connector. It is used on Ethernet twisted
pair links. This includes the 10Base-T, 100Base-TX, 100Base-T4, 100Base-T2, and
1000Base-T physical layer types. An RJ-45 connector has 8-pins. A male RJ-45 is
mounted on each end of the twisted pair cable. A female RJ-45 "jack" is integrated
into the Ethernet hub or NIC.
Cross Over
Color Code
Cable
Straight Through Cable Color Code
Pin 1
white green
Pin 1
white orange
Pin
2
green
Pin 2
orange
Pin
3
white orange
Pin 3
white green
Pin
4
blue
Pin 4
blue
Pin
5
white blue
Pin 5
white blue
Pin
6
orange
Pin 6
green
Pin
7
white brown
Pin 7
white brown
Pin
8
brown
Pin 8
brown
HUB :
Hubs provide the signal amplification required to allow a segment to be extended a
greater distance. A hub takes any incoming signal and repeats it out all ports. If the
hub is attached to a backbone, then all computers at the end of the twisted pair
segments can communicate with all the hosts on the backbone.
Hubs generally have no intelligence regarding the destination of the data they
propagate between the devices connected to it. Any data that is received at one port
is transmitted to all devices attached to the hub, including other hubs, which could,
in turn, then distribute the data to it's connected devices. So, only ONE network
connection
can
be
active
at
a
time
on
the
complete
network.
SWITCH:
Switches perform a similar function to hubs, but they have slightly more intelligence
and can accommodate devices with differing speeds. When data is sent through a
Hub, it is sent to every device attached to the Hub. A Switch sends the data only to
the destination device. A device connected to a switch gets the full bandwidth. In a
Hub, this bandwidth is shared by the connected devices. Thus, a 10Mbps signal will
be divided among say 10 computers in a Hub, but full 10Mbps will be delivered to the
destination port in a switch.
NETWORK INTERFACE CARD:
Network interface cards or NICs connect a PC to a network. The NIC provides a
physical connection between the networking cable and the computer's internal bus.
PCI bus master slots are most commonly used for NICs. NICs are usually 32 bit and
Plug & Play complaint. NICs are available to support almost all networking standards,
including the latest Fast Ethernet environment. Fast Ethernet NICs are often 10/100
capable, and will automatically set to the appropriate speed. Full duplex networking
is another option, where a dedicated connection to a switch allows a NIC to operate
at twice the speed, i.e. a 100Mbps connection will operate at 200Mbps.
Many NICs support WOL (Wake-On-LAN). This is an ACPI function allowing a
powered OFF computer to be powered ON from a remote station. To use the WOL
feature, the NIC must be WOL capable and the motherboard of the PC (for which the
card is installed) must be of ACPI architecture. The NIC will determine whether a
received Wake-up frame is addressed to the PC. If so, the NIC will send a signal to
the motherboard to power ON the computer.
TCP-IP
TCP/IP is an industry-standard suite of protocols. It is designed for large WANs.
TCP/IP was developed in 1969 by the U.S. Department of Defensethere was a project
called Advanced Research Projects Agency Network (ARPANET). The purpose of
TCP/IP was to provide high-speed communication network links. ARPANET has grown
into a worldwide community of networks known as the Internet.
TCP-IP protocol layers correspond to the layers of the OCI model.
Application Layer
Presentation Layer
TCP IP Protocols : HTTP, FTP,
Telnet, SMTP, DNS
Session Layer
Transport Layer
Transport Layer : TCP & UDP
Network Layer
IP, ARP, ICMP, IGMP
Data Link Layer
Network Interface Layer : Ethernet,
Token ring
Physical Layer
O S I Model
T C P / I P
Ethernet Addresses: Each Ethernet card (NIC) has a unique permanent 48 bit
address called MAC ( Media Access Control ) Address. This is hard coded into the
board. Each NIC in the world is unique in this respect. These are expressed as 12
Hex digits, for example: 0R45EBB67C11. Manufacturers of the NICs purchase blocks
of these addresses.
IP Addresses : Each node in a Network or specifically on the Internet must have a
unique 32 address. IP addresses operate at the Network layer of the TCP/IP protocol.
Theoretically, therefore, 4 billion unique host addresses can exist on the Internet. IP
addresses are expressed as a dotted decimal or dotted quad notation. Each number
is an Octet, i.e. consisting of 8 bits. For example: 192.168.8.30. This 32 bit, 4 octet
pattern consists of two parts: Network ID & Host ID.
11000000
192.
10101000
168.
00000011
3.
00011000 ----- Binary format
24
----- Dotted decimal format
Network ID or address identifies the Network on which the host exists, specifically, it
identifies the systems that are located on the same physical network. All systems on
the same physical network must have the same network ID. The network ID must be
unique to the internetwork.
Host ID or Host Address identifies the specific host on that network such as a
workstation, server, router, or other TCP/IP host. The address for each host must be
unique to the network ID.
The IP addresses are usually denoted in the form w.x.y.z
IP Classes:
To take care of the varying business needs, without wastage of IP addresses, IP
address classes were introduced, namely, A, B, C, D, E. Classes A, B & C use
different sizes for Network Ids & Host address part. Class D is for special type of
“multicast” addresses. Class E is experimental. Following tables illustrates each of
the A, B & C classes:
Clas
s
A
First
octet
value
(w)
1–126
Network
ID part
w
Host ID
part
Number
of
Networks
possible
x.y.z
126
Hosts per
Network
Application
16,777,214
For very large no.
of hosts
128–
191
192–
223
B
C
w.x
y.z
16,384
65,534
w.x.y
z
2,097,15
2
254
Medium to large
sized Networks
Small Networks
IP Ranges of Network & Host portions:
Network IDs
Host IDs
Clas
s
First
Last
First
Last
A
1.0.0.0
126.0.0.0
w.0.0.1
w.255.255.254
B
128.0.0.0
191.255.0.0
w.x.0.1
w.x.255.254
C
192.0.0.0
223.255.255.0
w.x.y.1
w.x.y.254
Subnets: In a large Intranet, parts of the total Network are required to be isolated.
More realistic number of Hosts per network than the ranges provided by the classes
of Networks are required. Too many number of Hosts should not be placed in a single
physical Network. When the Network is sub divided, there should be a way for a
computer to know whether the target computer is on the same Network OR the call
is to be routed. So, a setting is made in the TCP/IP configuration, called the Subnet
Mask. This helps to distinguish whether a host is in the same Subnet. Subnet mask
defines the bits of the TCP/IP Network part of addresses that should match, if the
target computer is to be considered on the same subnet. Subnets divide a large
network into multiple physical networks connected with routers. A subnet mask
blocks out part of the IP address so that TCP/IP can distinguish the network ID from
the host ID. When TCP/IP hosts try to communicate, the subnet mask determines
whether the destination host is on a local or remote network. To communicate on a
local network, computers must have the same subnet mask.
Default Gateway: When we require to communicate with a Host on another
Network, the call is to be “routed” to that Network by a device on this local network
that stores network IDs of other networks. For this the “Default Gateway” setting is
done on each host of the Local Network. TCP/IP sends packets for remote networks
to the default gateway, which then forwards the packets to the destination Network
or to other gateways until the packet is delivered to a gateway connected to the
specified destination.
Domain Name System ( DNS): It is a naming system used in TCP/IP networks to
translate computer names to Internet Protocol (IP) addresses. By refering to
computers by names, it is easy to locate computers & resources on the Network.
DNS services are available on the Server systems such as Windows Server 2003.
DOMAINS:
 Domain is a logical grouping of Network computers sharing a central directory
database. The directory database contains security information of the domain
such as user accounts.

Computers called Domain Controllers maintain the Domains with the help of
the Directory. All the security-related aspects of user and domain
interactions, centralizing security and administration are managed by the
Domain Controller.

Only Windows Server Operating systems of Windows 2000 or Windows 2003
Servers offer the services of the Domain Controller included in the “Active
Directory”.

Domains maintain a central list of users & passwords. These are
authenticated by the Domain controller each time a user tries to log in.

Users with different levels of powers, User and machine groups, Centralized
Security policies are some of the advanced services given by the Domain
controllers.
WORKGROUPS:

A logical grouping of computers in a Network is called a Workgroup. A
workgroup is said to exist in a Peer to Peer because all computers in the
workgroup can share resources as equals (peers) without a dedicated server.

Each computer in the workgroup handles the local security separately with a
local security database. which is a list of user accounts and resource security
information for the computer on which it resides.

If a user wants to access each computer in the network, he must have a User
account on it. Any changes to user accounts, such as changing a user's
password must be made on each computer in the workgroup for that account.

Workgroup is a convenient networking environment for a limited number of
computers in close proximity.
ACTIVE DIRECTORY

Active Directory (AD) is the directory Service which provides a place to store
information about network-based entities such as users, files, printers &
applications. It provides a means to locate and manage resources. Active
Directory provides a centralized and consistent way to manage, locate and
describe information about network resources.
 AD replicates directory data across a network so that data remains available if
one domain controller fails.
 The resources stored in the directory, such as user data, printers, servers,
groups, computers, and security policies, are known as objects. Active
Directory presents domain information in a hierarchical, object-based format
and protects network data from unauthorized access
 Active Directory supports clients running Windows XP Professional,
Windows 2000, Windows NT, and Windows 9x.
 To implement Active Directory, one or more DNS servers must be available to
the Windows 2003 domain, and the DNS client service must be configured at
each member computer. This is because Active Directory domains are named
with DNS names.
 Active Directory simplifies administration by providing a single point of
administration for all objects on the network.
Windows & Networking:
Windows Browser Service : Network Neighborhood OR My Network Places :
It gives Browse Services containing Browse Lists and Browse Servers OR Peer to
Peer Servers.









Any one or more Computers is a Browse Master. It maintains a list of the
Computers called Browse List – List of PCs in the Network – Having Win 98/ ME/
Win 2000/ XP. These act as Browse Servers.
Network Browse List allows to see all the Servers on the Network and from there
to see all the shares on the Servers.
The Browse Masters don’t distinguish between the Net Servers & Client Operating
Systems.
When one Computer starts, it gives a message on the Network like – “I am On &
who is the browse Masters”.
Then a Browse Master responds that “Pl. refer to me for Name service.”
In Win98—The peer to peer service is enabled by FILE & PRINTER SHARING.
In NT4, Win2000 / XP it is the SERVER Service.
Browse list will become unmanageable with thousands of computers.
Therefore form Work Groups.
One will get to see only the Servers on his Workgroup, at a time.
Each Workgroup will have its own Browse Master that is assigned automatically.
PCs not acting as Browse Servers will not appear in the Browse Lists.
Three basic software components are required for Windows Networking:
1. A Driver for the Network Interface Card
2. A Network Protocol
3. Network Client
Network Protocol software is included with Windows. TCP/IP is mostly used. Network
Client is the software component that can locate the Network resources and connects
to them. This Windows component is called “Client For Microsoft Networks”.
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