Introduction to Computer Networks
What can a Network do For You?
Introduction
A computer network can be two computers connected:
A computer network can also consist of, and is usually made for, more than two computers:
Characteristics of a Computer Network
The primary purpose of a computer network is to share resources:
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You can play a CD music from one computer while sitting on another computer
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You may have a computer with a CD writer or a backup system but the other computer doesn’t
have it; In this case, you can burn CDs or make backups on a computer that has one of these but
using data from a computer that doesn’t have a CD writer or a backup system

You may have a computer that doesn’t have a DVD player. In this case, you can place a movie
DVD on the computer that has a DVD player, and then view the movie on a computer that lacks a
DVD player
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You can connect a printer (or a scanner, or a fax machine) to one computer and let other
computers of the network print (or scan, or fax) to that printer (or scanner, or fax machine)

You can place a CD with pictures on one computer and let other computers access those pictures

You can create files and store them in one computer, then access those files from the other
computer(s) connected to it
Peer-to-Peer Networking
Based on their layout (not the physical but the imagined layout, also referred to as topology), there are
two types of networks. A network is referred to as peer-to-peer if most computers are similar and run
workstation operating systems:
It typically has a mix of Microsoft Windows 9X, Me, Windows XP Home Edition, or Windows XP
Professional (you can also connect a Novell SUSE Linux as part of a Microsoft Windows-based network;
the current release of the operating system is really easily to install and made part of the network).
In a peer-to-peer network, each computer holds its files and resources. Other computers can access
these resources but a computer that has a particular resource must be turned on for other computers to
access the resource it has. For example, if a printer is connected to computer A and computer B wants
to printer to that printer, computer A must be turned On.
Client/Server Networking
A computer network is referred to as client/server if (at least) one of the computers is used to "serve"
other computers referred to as "clients". Besides the computers, other types of devices can be part of
the network:
In a client/server environment, each computer still holds (or can still hold) its (or some) resources and
files. Other computers can also access the resources stored in a computer, as in a peer-to-peer
scenario. One of the particularities of a client/server network is that the files and resources are
centralized. This means that a computer, the server, can hold them and other computers can access
them. Since the server is always On, the client machines can access the files and resources without
caring whether a certain computer is On.
Another big advantage of a client/server network is that security is created, managed, and can highly
get enforced. To access the network, a person, called a user must provide some credentials, including a
username and a password. If the credentials are not valid, the user can be prevented from accessing
the network.
The client/server type of network also provides many other advantages such as centralized backup,
Intranet capability, Internet monitoring, etc.
In these series of lessons, the network we will build is based on Microsoft Windows operating systems (I
have been able to fully connect some versions of Linux, such as Novell SUSE Linux, into a Microsoft
Windows-based network but at the time of this writing, I will not be able to address that).
In our lessons, we will mention the names of companies or provide links. These are only indications and
not advertisements. Any other company or link that provides the mentioned service is suitable.
Network Distribution
Network Cables
Cable is used to connect computers. Although we are planning to use as much wireless as possible, you
should always have one or more cables around. In our network, we will use Category 5 cable RJ-45. The
ends of the cable appear as follows:
You can purchase this cable from a web store on the Internet. Probably the fastest way to get this cable
is to go to a computer store. When purchasing it, get something with at least 6ft.
Introduction to Network Distributors
In our introduction to networks, We mentioned that you could connect one computer to another. This
can be done using their serial ports:
This is possible because almost every computer has a serial port. If you have to connect many
computers to produce a network, this serial connection would not be practical. The solution is to use a
central object that the computers and other resources can connect to, and then this object becomes
responsible to “distribute” or manage network traffic:
The most regularly used types of network distributors are the hub, the router, and the switch.
Hub
A hub is rectangular box that is used as the central object on which computers and other devices are
connected. To make this possible, a hub is equipped with small holes called ports. Here is an example of
a hub:
Although this appears with 4 ports, depending on its type, a hub can be equipped with 4, 5, 12, or more
ports. Here is an example of a hub with 8 ports:
When configuring it, you connect an RJ-45 cable from the network card of a computer to one port of the
hub.
In most cases for a home-based or a small business network, you may not need a hub.
Routers: Wired or Wireless
Like a hub, a router is another type of device that acts as the central point among computers and other
devices that are part of a network. Here is an example of a wired router:
A router functions a little differently than a hub. In fact, a router can be considered a little "intelligent"
than the hub.
Like a hub, the computers and other devices are connected to a router using network cables. To make
this possible, a router is equipped with holes, called ports, in the back. Here is an example:
Based on advances in the previous years from IEEE and other organizations or research companies,
there are wireless routers. With this type, the computers and devices connect to the router using
microwaves (no physical cable).
In our (small) network, we wish to use a wireless router. Therefore, this is the kind we suggest you
purchase. You can purchase a wireless router from a computer store or on the internet
(http://www.tigerdirect.com, http://www.3com.com, http://www.provantage.com, etc). You
can also buy a wireless router from a computer store.
Wired Network Cards
In order to connect to a network, a computer must be equipped with a device called a network card. A
network card, or a network adapter, also called a network interface card, or NIC, allows a computer to
connect to the exterior. If you buy a computer from one of those popular stores or big companies on the
Internet, most of their computers have a network card tested and already. You can reliably use it. If you
go to a store that sells or manufactures computers, you can ask them to install or make sure that the
computer has a network card.
If you have a computer that doesn’t have a network card, you can install one. If you have a computer
that already has a network card, you can still replace it.
When it comes to their installation, there are roughly two categories of network cards: internal and
external. An internal network card looks like a printed circuit board with some objects "attached" or
"glued" to it and it appears as follows:
What this card looks like may not be particularly important and it may depend on the manufacturer but
some of its aspects particularly are. To start, there are two types of cards and you should know which
one is suited (or which one you want to use) for your computer. One type of NICs uses a peripheral
component interconnect (PCI) connection. Another type uses industry standard architecture (ISA).
There are two primary ways you replace a network card. In most cases, you will remove the card your
computer already has and install a new one. In some other cases, you will only add a new card but you
cannot replace the existing one because it is part of the motherboard (I have found that out lately when
opening a few computers for my users (I was not aware of that)). The area where you add a network
card is called a slot.
Wireless Network Cards
Depending on your network budget or your customer's, instead of using wired network cards, you can
use wireless ones. A wireless NIC appears as its wired counterpart. Here are two examples:
Overall, the physical installation of a wireless network card follows the same rules as that of a wired
NIC. They normally come with easy to follow instructions but it may be a good idea to install the
wireless network adapters after installing the wireless router. Also, it may be a good idea to purchase
the network cards and the wireless router from the same manufacturer.
Most desktop computers (workstations) come without a wireless network card. If you purchase a
computer from one of the big companies on the Internet, you can choose to have it shipped with a
wireless NIC. Some companies may propose to install it before shipping the computer. If you buy a
computer from a store and if you want to use wireless networking, you can buy a wireless network card
separately. As stated already, a wireless network card is not particularly difficult to install.
Besides the wireless network cards that can be installed inside the computer, you can use external
cards. These are installed using a USB port. Here is an example of a USB adapter:
These adapters, like most USB objects, are easy to connect and use. Like the other hardware parts,
when you connect these, the computer detects them and helps you get them ready for use.
Unlike desktop computers, most laptops nowadays come equipped with a wireless network card (in fact
most laptops today ship with both a wired and a wireless adapters). This means that, after purchasing
or acquiring a laptop, you should simply check whether it has a wireless adapter. The way you check
this depends on the laptop. Therefore, check its documentation. If your laptop happens not to a have a
wireless adapter and you want to use one, you have two main options. The classic style of adapter
appears as the following two examples:
This adapter is inserted on a side of the laptop. Normally, you would easily see its port as there is
usually only one that is suited for this type of card on the laptop. As you may guess, this card can be
inserted and removed at will.
Network Accessories
Printers
If you attach a printer to one computer and share it, when that computer is off, nobody can print. An
alternative is to purchase a network printer. That is, a printer that will directly connect to the network
and people can print to it any time. There are two types of printers in this case:

Some printers come equipped with a network card. In this case, as we will learn in Lesson 5, you
can use an RJ-45 cable to connect it to a router or a hub

Some printers are equipped for a parallel port. To connect them to a network, you can purchase
what is referred to as a print server (or a Jet-Direct card). The manufacturer of the printer can sell
it to you
If you are using a wireless network, you can purchase a wireless print server. This allows you to connect
almost any type of printer, with or without a network card, to the network. You can purchase a wireless
print server from a computer store or from a web store. It is usually easy to install as it comes with
easy-to-follow instructions.
Internet Service Provider (ISP)
An Internet Service Provider (ISP) is a company that serves as the intermediary between your network
(or you) and the Internet. If you plan to give access to the Internet to the members of your network,
you may need this type of company. You can start by checking with your local telephone company or
your local TV cable company.
Firewall
Firewall is a security measure that consists of protecting your network from intruders. This is primarily
important if you plan to connect your network to the Internet. There are two types of firewalls:
hardware and software.
For a small network, when buying a router, you can inquire as to whether it has a built-in firewall. Many
of them do. Alternatively, you can use or configure one of the computers of your network as a firewall.
Network Setup (Peer-To-Peer)
Physical Connections
Wired Networking
After installing the operating systems
on the computers that will primarily participate in the network, you
can "physically" connect the computers and the router. You can
start connecting the pieces whether the computers are on or off.
Practical Learning: Wiring the Network
1. Shut down all computers and the router (if necessary)
2. Turn on one computer you will use to setup the router
3. You router should have come equipped with a piece of paper or a brochure of just a few
pages that lists the instructions to follow to setup the router. One of the early instructions
may ask you to insert the CD that came with the router, in the CD drive and wait for the
instructions. Follow these instructions faithfully
4. After setting up and configuring the router, turn it off and turn off the computer you used
to set it up (this step is optional)
5. Connect each of the other computers to the router using an RJ-45 cable for each
connection:
If you had turned off (some of) the machines, first turn on the router. Then, after a few
seconds, turn on the computers. If you receive some messages indicating that a network was
detected, fine. If not, don't worry, we will check the network later.
Wireless Networking
If you plan to setup a wireless network using a wireless router, you will need to use one
computer to set it up.
Practical Learning: Wirelessly Connecting a Network
1. Start the computer you will use to setup the router (you should turn the others off):
2. Most, if not all, wireless routers come with very easy to follow instructions. Most of them
usually ask you to first insert the CD that accompanies the router, that is, before
physically installing the router. Consult the documentation (usually just one or a few
pieces of paper or a small brochure) and faithfully follow its CD's instructions. At one time,
the instructions would indicate to you when to connect the computer and the wireless
router. To do this, you will use a cable (usually supplied to you) to connect one end to the
computer and another end to the router:
3. Because the steps to perform depend on the router (or the manufacturer), we will let you
perform as described by their documentation
4. After installing and setting up the wireless router, turn it off and turn the computer off
5. If you didn't yet, install the wireless network card(s) on the other computer(s).
For any computer that doesn't have a wireless network card but has a wired network card,
connect it to a port of the wireless router using an RJ-45 cable. The computers that have a
network card will not need a physical connection to the wireless router:
6. Turn on the router. After a few seconds, turn on the computers one by one.
You may not need to check whether they work at this time or not. We will check this later
[edit] Introduction
A computer network allows sharing of resources and information among interconnected devices.
In the 1960s, the Advanced Research Projects Agency (ARPA) started funding the design of the
Advanced Research Projects Agency Network (ARPANET) for the United States Department of
Defense. It was the first computer network in the world. [1] Development of the network began in
1969, based on designs developed during the 1960s.
[edit] Purpose
Computer networks can be used for a variety of purposes:
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Facilitating communications. Using a network, people can communicate efficiently and easily via
email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.
Sharing hardware. In a networked environment, each computer on a network may access and use
hardware resources on the network, such as printing a document on a shared network printer.
Sharing files, data, and information. In a network environment, authorized user may access data and
information stored on other computers on the network. The capability of providing access to data and
information on shared storage devices is an important feature of many networks.
Sharing software. Users connected to a network may run application programs on remote
computers.
Information preservation.
Security.
Speed up.
[edit] Network classification
The following list presents categories used for classifying networks.
[edit] Connection method
Computer networks can be classified according to the hardware and software technology that is
used to interconnect the individual devices in the network, such as optical fiber, Ethernet,
wireless LAN, HomePNA, power line communication or G.hn.
Ethernet as it is defined by IEEE 802 utilizes various standards and mediums that enable
communication between devices. Frequently deployed devices include hubs, switches, bridges, or
routers. Wireless LAN technology is designed to connect devices without wiring. These devices
use radio waves or infrared signals as a transmission medium. ITU-T G.hn technology uses
existing home wiring (coaxial cable, phone lines and power lines) to create a high-speed (up
to 1 Gigabit/s) local area network.
[edit] Wired technologies

Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair cabling
consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated
copper wires twisted into pairs. Computer networking cabling consist of 4 pairs of copper cabling that
can be utilized for both voice and data transmission. The use of two wires twisted together helps to
reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2
million bits per second to 100 million bits per second. Twisted pair cabling comes in two forms which
are Unshielded Twisted Pair (UTP) and Shielded twisted-pair (STP) which are rated in categories which
are manufactured in different increments for various scenarios.

Coaxial cable is widely used for cable television systems, office buildings, and other worksites for
local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer
typically of a flexible material with a high dielectric constant, all of which are surrounded by a
conductive layer. The layers of insulation help minimize interference and distortion. Transmission
speed range from 200 million to more than 500 million bits per second.

Optical fiber cable consists of one or more filaments of glass fiber wrapped in protective layers. It
transmits light which can travel over extended distances. Fiber-optic cables are not affected by
electromagnetic radiation. Transmission speed may reach trillions of bits per second. The transmission
speed of fiber optics is hundreds of times faster than for coaxial cables and thousands of times faster
than a twisted-pair wire.[citation needed]
[edit] Wireless technologies

Terrestrial microwave – Terrestrial microwaves use Earth-based transmitter and receiver. The
equipment look similar to satellite dishes. Terrestrial microwaves use low-gigahertz range, which limits
all communications to line-of-sight. Path between relay stations spaced approx, 30 miles apart.
Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.

Communications satellites – The satellites use microwave radio as their telecommunications
medium which are not deflected by the Earth's atmosphere. The satellites are stationed in space,
typically 22,000 miles (for geosynchronous satellites) above the equator. These Earth-orbiting systems
are capable of receiving and relaying voice, data, and TV signals.

Cellular and PCS systems – Use several radio communications technologies. The systems are divided to
different geographic areas. Each area has a low-power transmitter or radio relay antenna device to
relay calls from one area to the next area.

Wireless LANs – Wireless local area network use a high-frequency radio technology similar to digital
cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to
enable communication between multiple devices in a limited area. An example of open-standards
wireless radio-wave technology is IEEE.

Infrared communication , which can transmit signals between devices within small distances not more
than 10 meters peer to peer or ( face to face ) without any body in the line of transmitting.
[edit] Scale
Networks are often classified as local area network (LAN), wide area network (WAN),
metropolitan area network (MAN), personal area network (PAN), virtual private network
(VPN), campus area network (CAN), storage area network (SAN), and others, depending on
their scale, scope and purpose, e.g., controller area network (CAN) usage, trust level, and
access right often differ between these types of networks. LANs tend to be designed for internal
use by an organization's internal systems and employees in individual physical locations, such as
a building, while WANs may connect physically separate parts of an organization and may include
connections to third parties.
[edit] Functional relationship (network architecture)
Computer networks may be classified according to the functional relationships which exist among
the elements of the network, e.g., active networking, client–server and peer-to-peer
(workgroup) architecture.
[edit] Network topology
Main article: Network topology
Computer networks may be classified according to the network topology upon which the
network is based, such as bus network, star network, ring network, mesh network.
Network topology is the coordination by which devices in the network are arranged in their logical
relations to one another, independent of physical arrangement. Even if networked computers are
physically placed in a linear arrangement and are connected to a hub, the network has a star
topology, rather than a bus topology. In this regard the visual and operational characteristics of a
network are distinct. Networks may be classified based on the method of data used to convey the
data, these include digital and analog networks.
[edit] Types of networks based on physical scope
Common types of computer networks may be identified by their scale.
[edit] Local area network
A local area network (LAN) is a network that connects computers and devices in a limited
geographical area such as home, school, computer laboratory, office building, or closely
positioned group of buildings. Each computer or device on the network is a node. Current wired
LANs are most likely to be based on Ethernet technology, although new standards like ITU-T
G.hn also provide a way to create a wired LAN using existing home wires (coaxial cables, phone
lines and power lines).[2]
Typical library network, in a branching tree topology and controlled access to resources
All interconnected devices must understand the network layer (layer 3), because they are
handling multiple subnets (the different colors). Those inside the library, which have only 10/100
Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to the central
router, could be called "layer 3 switches" because they only have Ethernet interfaces and must
understand IP. It would be more correct to call them access routers, where the router at the top
is a distribution router that connects to the Internet and academic networks' customer access
routers.
The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their
higher data transfer rates, smaller geographic range, and no need for leased telecommunication
lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s.
This is the data transfer rate. IEEE has projects investigating the standardization of 40 and 100
Gbit/s.[3]
[edit] Personal area network
A personal area network (PAN) is a computer network used for communication among
computer and different information technological devices close to one person. Some examples of
devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs,
scanners, and even video game consoles. A PAN may include wired and wireless devices. The
reach of a PAN typically extends to 10 meters.[4] A wired PAN is usually constructed with USB and
Firewire connections while technologies such as Bluetooth and infrared communication typically
form a wireless PAN.
[edit] Home area network
A home area network (HAN) is a residential LAN which is used for communication between
digital devices typically deployed in the home, usually a small number of personal computers and
accessories, such as printers and mobile computing devices. An important function is the sharing
of Internet access, often a broadband service through a CATV or Digital Subscriber Line (DSL)
provider. It can also be referred to as an office area network (OAN).
[edit] Wide area network
A wide area network (WAN) is a computer network that covers a large geographic area such as
a city, country, or spans even intercontinental distances, using a communications channel that
combines many types of media such as telephone lines, cables, and air waves. A WAN often uses
transmission facilities provided by common carriers, such as telephone companies. WAN
technologies generally function at the lower three layers of the OSI reference model: the
physical layer, the data link layer, and the network layer.
[edit] Campus network
A campus network is a computer network made up of an interconnection of local area networks
(LAN's) within a limited geographical area. The networking equipments (switches, routers) and
transmission media (optical fiber, copper plant, Cat5 cabling etc.) are almost entirely owned (by
the campus tenant / owner: an enterprise, university, government etc.).
In the case of a university campus-based campus network, the network is likely to link a variety
of campus buildings including; academic departments, the university library and student
residence halls.
Sample VPN used to interconnect 3 offices and remote users
[edit] Virtual private network
A virtual private network (VPN) is a computer network in which some of the links between
nodes are carried by open connections or virtual circuits in some larger network (e.g., the
Internet) instead of by physical wires. The data link layer protocols of the virtual network are said
to be tunneled through the larger network when this is the case. One common application is
secure communications through the public Internet, but a VPN need not have explicit security
features, such as authentication or content encryption. VPNs, for example, can be used to
separate the traffic of different user communities over an underlying network with strong security
features.
VPN may have best-effort performance, or may have a defined service level agreement (SLA)
between the VPN customer and the VPN service provider. Generally, a VPN has a topology more
complex than point-to-point.
[edit] Internetwork
An internetwork is the connection of two or more private computer networks via a common
routing technology (OSI Layer 3) using routers. The Internet is an aggregation of many
internetworks, hence its name was shortened to Internet.
[edit] Global Area Network
A Global Area Network (GAN) is a network used for supporting mobile communications across
an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile
communications is handing off the user communications from one local coverage area to the
next. In IEEE Project 802, this involves a succession of terrestrial wireless LANs.[5]
[edit] Internet
The Internet is a global system of interconnected governmental, academic, corporate, public,
and private computer networks. It is based on the networking technologies of the Internet
Protocol Suite. It is the successor of the Advanced Research Projects Agency Network
(ARPANET) developed by DARPA of the United States Department of Defense. The Internet
is also the communications backbone underlying the World Wide Web (WWW).
Participants in the Internet use a diverse array of methods of several hundred documented, and
often standardized, protocols compatible with the Internet Protocol Suite and an addressing
system (IP addresses) administered by the Internet Assigned Numbers Authority and
address registries. Service providers and large enterprises exchange information about the
reachability of their address spaces through the Border Gateway Protocol (BGP), forming a
redundant worldwide mesh of transmission paths.
[edit] Intranets and extranets
Intranets and extranets are parts or extensions of a computer network, usually a local area
network.
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web
browsers and file transfer applications, that is under the control of a single administrative entity.
That administrative entity closes the intranet to all but specific, authorized users. Most
commonly, an intranet is the internal network of an organization. A large intranet will typically
have at least one web server to provide users with organizational information.
An extranet is a network that is limited in scope to a single organization or entity and also has
limited connections to the networks of one or more other usually, but not necessarily, trusted
organizations or entities—a company's customers may be given access to some part of its
intranet—while at the same time the customers may not be considered trusted from a security
standpoint. Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type
of network, although an extranet cannot consist of a single LAN; it must have at least one
connection with an external network.
[edit] Overlay network
An overlay network is a virtual computer network that is built on top of another network. Nodes
in the overlay are connected by virtual or logical links, each of which corresponds to a path,
perhaps through many physical links, in the underlying network.
[edit] Basic hardware components
All networks are made up of basic hardware building blocks to interconnect network nodes, such
as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. In addition, some
method of connecting these building blocks is required, usually in the form of galvanic cable
(most commonly Category 5 cable). Less common are microwave links (as in IEEE 802.12) or
optical cable ("optical fiber").
[edit] Network interface cards
A network card, network adapter, or NIC (network interface card) is a piece of computer
hardware designed to allow computers to communicate over a computer network. It provides
physical access to a networking medium and often provides a low-level addressing system
through the use of MAC addresses.
Each network interface card has its unique id. This is written on a chip which is mounted on the
card.
[edit] Repeaters
A repeater is an electronic device that receives a signal, cleans it of unnecessary noise,
regenerates it, and retransmits it at a higher power level, or to the other side of an obstruction,
so that the signal can cover longer distances without degradation. In most twisted pair Ethernet
configurations, repeaters are required for cable that runs longer than 100 meters. Repeaters
work on the Physical Layer of the OSI model.
[edit] Hubs
A network hub contains multiple ports. When a packet arrives at one port, it is copied
unmodified to all ports of the hub for transmission. The destination address in the frame is not
changed to a broadcast address.[7] It works on the Physical Layer of the OSI model..
[edit] Bridges
A network bridge connects multiple network segments at the data link layer (layer 2) of the
OSI model. Bridges broadcast to all ports except the port on which the broadcast was received.
However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC
addresses are reachable through specific ports. Once the bridge associates a port and an
address, it will send traffic for that address to that port only.
Bridges learn the association of ports and addresses by examining the source address of frames
that it sees on various ports. Once a frame arrives through a port, its source address is stored
and the bridge assumes that MAC address is associated with that port. The first time that a
previously unknown destination address is seen, the bridge will forward the frame to all ports
other than the one on which the frame arrived.
Bridges come in three basic types:
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Local bridges: Directly connect local area networks (LANs)
Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote
bridges, where the connecting link is slower than the end networks, largely have been replaced with
routers.
Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
[edit] Switches
A network switch is a device that forwards and filters OSI layer 2 datagrams (chunks of data
communication) between ports (connected cables) based on the MAC addresses in the packets.[8]
A switch is distinct from a hub in that it only forwards the frames to the ports involved in the
communication rather than all ports connected. A switch breaks the collision domain but
represents itself as a broadcast domain. Switches make forwarding decisions of frames on the
basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for
devices, and cascading additional switches.[9] Some switches are capable of routing based on
Layer 3 addressing or additional logical levels; these are called multi-layer switches. The term
switch is used loosely in marketing to encompass devices including routers and bridges, as well
as devices that may distribute traffic on load or by application content (e.g., a Web URL
identifier).
[edit] Routers
A router is an internetworking device that forwards packets between networks by processing
information found in the datagram or packet (Internet protocol information from Layer 3 of the
OSI Model). In many situations, this information is processed in conjunction with the routing
table (also known as forwarding table). Routers use routing tables to determine what interface to
forward packets (this can include the "null" also known as the "black hole" interface because data
can go into it, however, no further processing is done for said data).
[edit] Firewalls
Firewalls are the most important aspect of a network with respect to security. A firewalled system
does not need every interaction or data transfer monitored by a human, as automated processes
can be set up to assist in rejecting access requests from unsafe sources, and allowing actions
from recognized ones. The vital role firewalls play in network security grows in parallel with the
constant increase in 'cyber' attacks for the purpose of stealing/corrupting data, planting viruses,
etc.