Computer networks

Lecture#6Date:

Computer Network

• Any system of interconnected computers, computer

peripheral equipments used for transmit and receive

of information.

Applications of Networks

• Resource Sharing– Hardware (computing resources, disks, printers)– Software (application software)

• Information Sharing– Easy accessibility from anywhere (files, databases)– Search Capability (WWW)

• Communication– Email– Message broadcasting

Components of a Computer

Network

Components of a Network

A computer network typically includes four things:

1. Network interface cards (NICs)

2. Networking Software

3. Networking Media

4. Networking Devices

1) Network interface cards (NICs):

• Plugs into the back of your computers and lets them send and receive messages from other computers

• The MAC address is a 48 bit long unique value associated with a network adapter.

• MAC addresses are also known as hardware addresses or physical addresses.

IP Address

• Every machine on the Internet has a unique identifying number, called an IP Address.

• A typical IP address looks like this:

216.27.61.137

• Computers communicate in binary form. Look at the same IP address in binary:

11011000.00011011.00111101.10001001

2) Networking Software

TCP/IP • The basic communication

language or protocol of the Internet.

• The TCP/IP model has four layers.

• Each layer performs a certain role or task.

• Each layer a packet of information travels through adds what is called a header.

TCP/IP Data Communication Example

Network Header

SegmentHeader

FrameTrailerData

Segment Header Data

Data

FrameHeader

NetworkHeader

Segment Header Data

0111111010101100010101101010110001

E-mail message

Data

Segment

Packet

Frame(medium dependent)

Bits

3) Networking Devices

All networks are made up of basic hardware building blocks to interconnect network nodes, such as:

• Hubs• Switches• Routers

Building a Networking using Hub

Hub

Building a Networking using Switch

Segmentation with Switching

Switches

Building an Internetworking using Router

• An Internetwork is a collection of individual networks,

connected by intermediate networking devices, that

function as a single large network.

• A router is needed to connect individual networks.

• Router maintains routing table to keep track of individual

networks.

Routers

Networking Media

• Networking media can be defined simply as the

means by which signals (data) are sent from one

computer to another (either wired or wireless

means).

Networking MediaPhysical communications Channels

• Twisted Pair

– uses copper wire, twisted together in pairs

to form a telephone wire.

– Maximum segment length of 100 meters

• Coaxial Cable/Co-ax

– uses one or more central wire conductors

surrounded by an insulator and encased in

metal sheathing.

Networking Media• thin coaxial cable is able to transport a signal up to

around 185 meters without line loss.

• thick coaxial cable is able to carry signals over long distances: up to 500 meters without line loss

• Fiber-optic Cable– A physical communications channel that uses light and

glass fibers. – It is light-weight and tolerates data rates on the order

of 100Mbps up to 60km.

Networking MediaWireless Channels

Wireless channels transmit data through air or space

• Radio Waves– RF frequency ranges from roughly 30 kilohertz to 1 gigahertz– A radio wave is an electromagnetic wave propagated by an

antenna. – All FM radio stations transmit in a band of frequencies

between 88 megahertz and 108 megahertz.

• Microwave– Microwaves frequency ranges between 1 GH(109 Hz) to 1 TH

(1012 Hz).

Networking Media• They are the principal carriers of television, telephone, and

data transmissions between stations on Earth and between the Earth and satellites.

• Radar beams are short pulses of microwaves used to locate ships and planes etc.

• Infrared– Transmits data and information by means of an infrared

light. The signal cannot travel through objects.– commonly used in remote control of TVs, VCRs and CD

players

Network Topology

Network Topology

• The network topology defines the way in which computers, printers, and other devices are connected.

• LAN topologies define the manner in which network devices are organized.

Introduction to Computer Networks

Bus Topology

• Commonly referred to as a linear bus,

all the devices on a bus topology are

connected by one single cable.

• Broadcast shared media.

• Suitable for small networks.

• Single point of failure.

Introduction to Computer Networks

Star & Tree Topology

Introduction to Computer Networks

• The star topology is the most commonly used

architecture in most LANs.

• No single point-of-failure

• Larger networks use the extended star

topology also called tree topology.

• When used with network devices, like bridges,

switches, and routers, this topology

significantly reduces the traffic on the wires by

sending packets only to the wires of the

destination host.

Ring TopologyIntroduction to Computer Networks

• A frame travels around the ring, stopping at each

node. If a node wants to transmit data, it adds the

data as well as the destination address to the frame.

• The frame then continues around the ring until it

finds the destination node, which takes the data out

of the frame.

• The dual ring topology allows data to be sent in both

directions.

Mesh Topology

• The mesh topology connects all devices (nodes)

to each other for redundancy and fault

tolerance.

• Chances of connection break downs are very

minimal.

• It is used in WANs to interconnect LANs and for

mission critical networks like those used by

banks and financial institutions.

• Implementing the mesh topology is expensive

and difficult.

Introduction to Computer Networks

Network Classification

Network Classification• Based on their size, networks

can be classified as:

• Local Area Network (LAN)

• Metropolitan Area Network

(MAN)

• Wide Area Network (WAN)

• Internet

• A network covering a small geographic area, like a home, office, or building.

• LANs are designed to share resources between personal computers.

• The resources to be shared can include hardware (e.g., printer, Scanner), software (e.g., an application program) or data.

Local Area Network (LAN)

• A network that connects two or more LANs but does not extend

beyond the boundaries of the immediate town/city.

• In many cities under the streets, Copper and Fiber Optic cables are

run. Above ground we find Wireless, Microwave and Lasers. These

technologies are typically limited to distances under 100 miles.

These limitations define a MAN or Metropolitan Area Network.

Examples:

• A network of fire stations in a suburban area would be an example

• A newspaper/media company employ a MAN.

• A chain of community colleges could be linked by a MAN.

Metropolitan Area Network (MAN)

• A data communications network that covers a relatively

broad geographic area (i.e. one city to another and one

country to another).

• WAN often uses transmission facilities provided by

common carriers, such as telephone companies.

• Dedicated transoceanic cabling or satellite uplinks may

be used.

Wide Area Network (WAN)

WAN

Modern internetwork

• There are at least three variants of internetwork, depending on who administers and who participates in them:

• Intranet • Extranet • VPN

Modern Internetwork

What is an Intranet?

• Intranet is an internal network used strictly within the

confines of a company, university, or organization.

• Belongs to an organization, and is designed to be accessible

only by the organization's members, employees, or others

with authorization.

• An intranet's Web site looks and act just like other Web sites,

but has a firewall surrounding it to guard against

unauthorized users.

What is an Extranet?

• An “extended intranet”, uses the Internet to link selected resources of a company’s intranet with its customers, suppliers, or other business partners

• Extranet transmissions are conducted over the Internet to save money. But it offers no privacy or transmission security.

• By creating tunnels of secure data flows using cryptography and authorization algorithms, called VPNs, the security can be improved.

Virtual private network (VPN)• Provide secure connectivity between a corporation’s intranets and

intranets of its business partners, material suppliers, financial services, and customers.

• Tunneling: the process by which VPNs transfer information by encapsulating traffic in IP packets over the Internet

Layered System View

Clients, partners, customers

Intranet

Extranet

Internet

Corporate members

Global society: competitors

Network security

Common Network Attacks• Reading contents of messages is called eavesdropping• Difficult to detect• Sniffing/Snooping - Monitoring the network for sensitive

data and passwords

Common Network Attacks

• Modification or creation of messages (by attackers)

• Easy to detect but difficult to prevent

Common Network AttacksDenial-of-service Attack- Lowering or removing quality of service in a network

Common Network Attacks

Message Replays - Sending a message repeatedly to a receiver

Security MethodsFirewalls• A firewall is a part of a network that is designed to block unauthorized

access while permitting authorized communications.

• It is a device or set of devices that is configured to permit or deny network transmissions based upon a set of rules and other criteria.

• All messages entering or leaving the intranet pass through the firewall.

• Firewalls can’t protect against malicious insiders viruses, and scans for source & destination addresses & port numbers, not details of data

Security Methods• Password-based Authentication

• Address-based Authentication (network address)

• Personal Tokens (hardware & password/ smart cards)

• Biometrics (fingerprint, voiceprint, handwriting)

• Antivirus software