Introduction to Computer Networks
What can a
do For You?
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:
You can play a CD music from one computer while sitting on another computer
You may have a computer with a CD writer or a backup system but the other computer doesn’t
have it; fn this caseI you can burn Cas or make
on a computer that has one of these bu
using dat a from a comput er t hat doesn’t have a CD writ er or a backup syst em
You may have a comput er t hat doesn’t have a DVD player. In t his case, you can place a movie
asa on t he comput er t hat has a asa playerI and t hen view t he movie on a comput er t hat
You can connect a printer (or a scanner, or a fax machine) to one computer and let other
computers of the network
(or scan, or fax) to that printer (or scanner, o
r 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
Ba s e d o n t he i r l a y o ut ( no t t he p hy s i c a l b ut t he i ma g i ne d l a y o ut, a l s o r e f e r r e d t o a s t o p o l o g y ), t he r e a r e
t wo t y p e s o f ne t wo r ks. A ne t wo r k i s r e f e r r e d t o a s p e e r
p e e r i f mo s t c o mp u t e r s a r e s i mi l a r a n d r u n
w o r k s t a t i o n
o p e r a t i n g s y s t e ms
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
the current release of the operating system is really easily to install and made part of the network).
In a peer
peer net work, each comput er holds it s files and resources. Ot her comput ers can access
t hese resources but a
comput er t hat has a part icular resource must be t urned on for ot her comput ers t o
access t he resource it has. For example, if a print er is connect ed t o comput er A and comput er B want s
t o print er t o t hat print er, comput er A must be t urned On.
A comput er net work is referred t o as client/server if (at least ) one of t he comput ers is used t o "serve"
ot her comput ers referred t o as "client s". Besides t he comput ers, ot her t ypes of devices can be part of
t he net work:
In a client/server environment, each comput er st ill holds (or can st ill hold) it s (or some) resources and
files. Ot her comput ers can also access t he resources st ored in a comput er, as in a peer
scenario. One of t he part icularit ies of a client/serve
r net work is t hat t he files and resources are
cent ralized. This means t hat a comput er, t he
, can hold t hem and ot her comput ers can access
t hem. Since t he server is always
On, t he client machines can access t he files and resources wit hout
caring whet her a cert ain comput er is On.
Anot her big advant age of a client/server net work is t hat securit y is creat ed, managed, and can highly
get enforced. To access t he net work, a person
, called a user must provide some credent ials, including a
username and a password. If t he credent ials are not valid, t he user can be prevent ed from accessing
t he net work.
The client/server t ype of net work also provides many ot her advant ages such as cent ra
Int ranet capabilit y, Int ernet monit oring, et c.
In t hese series of lessons, t he net work we will build is based on Microsoft Windows operat ing syst ems (I
have been able t o fully connect some versions of Linux, such as Novell SUSE Linux, int o a
based net work but at t he t ime of t his writ ing, I will not be able t o address t hat ).
In our lessons, we will ment ion t he names of companies or provide links. These are only indicat ions and
not advert isement s. Any ot her company or link t hat
provides t he ment ioned service is suit able.
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
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
Introduction to Network Distributors
to networks, We mentioned that you could connect one computer to another. This
can be done using their serial ports:
This is possib
le 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, an
d 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.
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
Although this appears with 4 ports, depending on its type, a hub can be equipped with 4, 5, 12, or more
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
In most cases for a home
based or a small business network, you may not need a hub.
Routers: Wired or W
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 ro
uter 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 w
ish 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
, 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
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
l. 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 aspe
cts 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 st
andard 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 ro
uter 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
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 bo
th 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 happen
s 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 the
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.
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 netwo
rk 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 J
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
Internet Service Provider (ISP)
An Internet Service Provider (ISP) is a company that serves as th
e 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
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.
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
After installing the
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
Shut down all comp
uters and the router (if necessary)
Turn on one computer you will use to setup the router
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 e
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
After setting up and configuring the router, turn it off and turn off the computer you used
t it up (this step is optional)
Connect each of the other computers to the router using an RJ
45 cable for each
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
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
Start the computer you will use to setup the router (you should turn the others off):
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 con
nect 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:
Because the steps to perform depend on the router (or the manufacturer), we will let you
perform as described by their documentation
After installing and setting up the wireless router, turn it off and turn the computer off
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
. The computers that have a
network card will not need a physical connection to the wireless router:
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
A computer network allows sharing of resources and info
rmation among interconnected devices.
In the 1960s, the Advanced Research Projects Agency (
) started funding the design of the
Advanced Research Projects Agency Network (
) for the United States Department of
Defense. It was the first computer network in the world.
t of the network began in
1969, based on designs developed during the 1960s.
Computer networks can be used for a variety of purposes:
Using a network, people can communicate efficiently and easily via
email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.
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 ne
twork. The capability of providing access to data and
information on shared storage devices is an important feature of many networks.
Users connected to a network may run application
The following list presents categories used for classifying networks.
uter networks can be classified according to the hardware and software technology that is
used to interconnect the individual devices in the network, such as
power line communication
Ethernet as it is defined by IEEE 802 utilizes various s
tandards 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
signals as a transmission medium.
, phone lines and
) to create a high
to 1 Gigabit/s) local area network.
is the most widely used medium for telecommunication. Twisted
ist 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
. 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.
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 o
f 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.
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. Transmiss
ion 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
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
Path between relay stations spaced approx, 30 miles apart.
Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.
ellites 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
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 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
wave technology is IEEE.
, 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.
Networks are often classified as
local area network
wide area network
politan area network
personal area network
ual private network
campus area network
storage area network
nd 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 par
Functional relationship (network architecture)
Computer networks may be classifi
ed according to the functional relationships which exist among
the elements of the network, e.g.,
Computer networks may be classified according to the
upon which the
network is based, such as
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
data, these include digital and analog networks.
Types of networks based on physical scope
Common types of computer networks m
ay be identified by their scale.
Local area network
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 ne
twork is a node. Current wired
LANs are most likely to be based on
technology, although new standards like
also provide a way to create a wired LAN using existing home wires (coaxial cables, phone
lines and power lines).
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 Etherne
t connection to the central
router, could be called "layer 3 switches" because they only have Ethernet interfaces and must
. It would be more correct to ca
ll them access routers, where the router at the top
is a distribution router that connects to the Internet and academic networks' customer access
The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their
data transfer rates, smaller geographic range, and no need for leased telecommunication
lines. Current Ethernet or other
LAN technologies operate at speeds up to 10 Gbit/s.
his is the data transfer rate.
has projects investigating the standardization of 40 and 100
Personal area network
personal area network
(PAN) is a compute
r 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.
A wired PAN is usually constructed with USB and
nnections while technologies such as Bluetooth and infrared communication typically
form a wireless PAN.
Home area network
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 p
rinters and mobile computing devices. An important function is the sharing
of Internet access, often a broadband service through a CATV or
provider. It can also be referred to as an office area network (OAN).
Wide area network
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 teleph
one 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
data link layer
, and the
is a computer network made up of an interconnection of local area networks
(LAN's) within a limited geographical area. The networking equipments (switches, rou
transmission media (optical fiber, copper plant,
cabling etc.) are almost entirely owned (by
the campus tenant / owner: an enterprise, university, government
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
Sample VPN used to interconnect 3 offices
and remote users
Virtual private network
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 lay
er 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 authentic
ation or content encryption. VPNs, for example, can be used to
separate the traffic of different user communities over an underlying network with strong security
VPN may have best
effort performance, or may have a defined service level agreement
between the VPN customer and the VPN service provider. Generally, a VPN has a topology more
complex than point
is t he connect ion of t wo or more privat e comput er net works via a common
rout ing t echnology (OSI
) using rout ers. The Int ernet is an aggregat ion of many
int ernet works, hence it s name was short ened t o Int ernet.
Global Area Network
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 cha
llenge 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
is a global system of interconnected governmental, academic, corporate, public,
and private computer networks. It is based on the networking technologies of the
. It is the successor of the
Advanced Research Projects Agency Network
(ARPANET) developed by
United States Department of Defense
. The Internet
is also the communications backbone underlying the
World Wide Web
Participants in the Internet use a diverse array of methods of several hundred documented, and
tandardized, protocols compatible with the Internet Protocol Suite and an addressing
) administered by the
Internet Assigned Numbers Authority
. Service providers and large enterprises exchange inform
ation about the
of their address spaces through the
Border Gateway Protocol
P), forming a
redundant worldwide mesh of transmission paths.
Intranets and extranets
Intranets and extranets are parts or extensions of a computer
network, usually a local area
is a set of networks, using the
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 inte
rnal network of an organization. A large intranet will typically
have at least one web server to provide users with organizational information.
is a network that is limited in sc
ope 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
while at the
same time the customers may not be considered
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.
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.
Basic hardware components
All networks are made up of basic hardware building blocks to interconnect network
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
Category 5 cable
). Less common are microwave links (as in
optical cable ("
Network interface cards
, network adapter, or NIC (network interface card) is a piece of
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
Each network interface card has its unique
id. This is written on a chip which is mounted on the
device that receives a
, cleans it of u
regenerates it, and
it at a higher power level, or to the other side of an obstruction,
so that the signal can cov
er 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.
contains multiple ports. When a packet arrives at one port, it is copied
all ports of the hub for transmission. The destination address in the frame is not
changed to a broadcast address.
It works on the Physical Layer of the OSI model..
data link layer
(layer 2) of the
. 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
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 b
y 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 destina
tion 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:
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
Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
is a device that forwards and filters
OSI layer 2
(chunks of data
communication) between ports (connected cables) based on the MAC addresses in
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 t
he 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.
Some switches are capable of routing based on
Layer 3 addressing or additional logical levels; these are called multi
layer switches. The term
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
is an internetworking device that forwards
between networks by processing
information found in the datagram or packet (Internet protocol information from
Layer 3 of the
). 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
ts (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).
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 a
ssist 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,