NetworksNetworks

Software Development Unit 2 2010

What is a network?What is a network?

A network is two A network is two or more or more computers computers connected connected together for the together for the purpose of:purpose of: sharing resources sharing resources

(printers, files, (printers, files, software, software, hardware)hardware)

communicationcommunication

How are they classified?How are they classified?

Their size: LAN, WANTheir size: LAN, WAN Their configuration: client-server, peer-Their configuration: client-server, peer-

to-peer (no servers)to-peer (no servers) The rules they use to exchange data: The rules they use to exchange data:

protocolsprotocols How they are linked together: cable, How they are linked together: cable,

wirelesswireless Their logical shape (topology): bus, tree, Their logical shape (topology): bus, tree,

star, ring, meshstar, ring, mesh

Types of networks:Types of networks:

Local Area Networks Local Area Networks (LAN)(LAN) One locationOne location

Wide Area Networks Wide Area Networks (WAN)(WAN) Centralised WAN: Centralised WAN:

Servers in one locationServers in one location Distributed WAN: Distributed WAN:

Servers distributed Servers distributed throughout the throughout the network (the Internet network (the Internet is an example of this)is an example of this)

LAN

WAN

Network configurationsNetwork configurations Client-serverClient-server

File serverFile server Files stored and accessed Files stored and accessed

from central locationfrom central location Application serverApplication server

Applications installed and Applications installed and executed from central executed from central location.location.

Often used in combination Often used in combination with file serverswith file servers

Peer-to-peerPeer-to-peer No serversNo servers Applications installed on Applications installed on

each PC, files shared each PC, files shared

ProtocolsProtocols

A protocol is a convention A protocol is a convention or a standard that controls or a standard that controls or enables the connection, or enables the connection, communication and data communication and data transfer between transfer between computers on a networkcomputers on a network

Contains rules governing Contains rules governing the syntax, semantics and the syntax, semantics and synchronization of synchronization of communicationcommunication

May be implemented by May be implemented by hardware, software or a hardware, software or a combinationcombination

Like two people agreeing to Like two people agreeing to speak the same language…speak the same language…

Communication protocolsCommunication protocols

Communication protocols are Communication protocols are specifically designed to control data specifically designed to control data transfer over a communications channeltransfer over a communications channel

Some common examples:Some common examples: TCP/IP: Two protocols that control TCP/IP: Two protocols that control

communication across the Internet communication across the Internet and some networksand some networks

POP3, IMAP, SMTP: Protocols that can POP3, IMAP, SMTP: Protocols that can be used to send and receive emailbe used to send and receive email

HTTP: Method of publishing and HTTP: Method of publishing and retrieving web pagesretrieving web pages

FTP: Protocol for sending and FTP: Protocol for sending and receiving filesreceiving files

These examples are These examples are often referred to as the often referred to as the ‘Internet protocols’‘Internet protocols’

Network ProtocolsNetwork Protocols

Rules and conventions Rules and conventions for communication for communication between network between network devicesdevices

The commonly used The commonly used are are Ethernet, Wi-FiEthernet, Wi-Fi

Others exist but are Others exist but are not as widely usednot as widely used

EthernetEthernet Uses an access method called Uses an access method called

CSMA/CD (Carrier Sense Multiple CSMA/CD (Carrier Sense Multiple Access/Collision Detection)Access/Collision Detection) CS – Computers (or nodes) ‘listen’ CS – Computers (or nodes) ‘listen’

to the network to see if there are to the network to see if there are any signals being transmittedany signals being transmitted

MA – All computers (or nodes) can MA – All computers (or nodes) can ‘hear’ what is being transmitted on ‘hear’ what is being transmitted on the network at any one timethe network at any one time

CD – Collision detection: two CD – Collision detection: two computers (or nodes) could computers (or nodes) could transmit at the same time, causing transmit at the same time, causing a collisiona collision

EthernetEthernet What happens when there is a What happens when there is a

collision?collision? Each computer (or node) waits a Each computer (or node) waits a

random amount of time and then random amount of time and then retransmitsretransmits

The random time delay is usually The random time delay is usually enough to ensure the collision enough to ensure the collision does not reoccurdoes not reoccur

The Ethernet specified speed The Ethernet specified speed is 10Mb/sis 10Mb/s

The Fast Ethernet standard The Fast Ethernet standard increases this to 100Mb/sincreases this to 100Mb/s

Wi-FiWi-Fi

Based on the 802.11x Based on the 802.11x standardstandard

Cannot do collision Cannot do collision detection and instead detection and instead uses RTS/CTSuses RTS/CTS RTS: Request to send – RTS: Request to send –

packet sent by the packet sent by the sendersender

CTS: Clear to send – CTS: Clear to send – packet sent by the packet sent by the intended recipient that intended recipient that alerts all other nodes to alerts all other nodes to be ‘quiet’be ‘quiet’

TCP/IPTCP/IP

TCP/IP consists of two TCP/IP consists of two protocols: TCP and IP protocolprotocols: TCP and IP protocol TCP (Transport Control Protocol)TCP (Transport Control Protocol)

Used to divide data that needs to be Used to divide data that needs to be transmitted into a number of transmitted into a number of packets, each with a sequence packets, each with a sequence numbernumber

The TCP at the destination address The TCP at the destination address assembles the data and notifies the assembles the data and notifies the sender if any packets are not sender if any packets are not received within a certain time received within a certain time (‘timeout’)(‘timeout’)

IP (Internet Protocol)IP (Internet Protocol) Defines how information gets sent Defines how information gets sent

between network nodes between network nodes

TCP/IP: IP AddressTCP/IP: IP Address

In order for packets to be able In order for packets to be able to be sent across the Internet to be sent across the Internet or a network, each node must or a network, each node must have a unique IP address (just have a unique IP address (just like a street address)like a street address)

Domain Name Servers (DNS) Domain Name Servers (DNS) translate URLs to IP addressestranslate URLs to IP addresses

An IP address consists of 4 An IP address consists of 4 bytes (32 bits)bytes (32 bits) For example: 255.255.255.255For example: 255.255.255.255

TCP/IP: PacketsTCP/IP: Packets TCP divides data to be TCP divides data to be

sent into packetssent into packets Each packet contains:Each packet contains:

A header consisting of:A header consisting of: a destination addressa destination address a source addressa source address the packet numberthe packet number the total number of packets the total number of packets a checksuma checksum and some other things…and some other things…

The data itself (known as The data itself (known as the payload)the payload)

A trailerA trailer

TCP/IP: From source to TCP/IP: From source to destinationdestination

If a packet arrives at a If a packet arrives at a destination with an incorrect destination with an incorrect checksum, it is discardedchecksum, it is discarded

The path from the source to The path from the source to destination is in flux, therefore destination is in flux, therefore packets ‘get lost’, are packets ‘get lost’, are corrupted or are duplicatedcorrupted or are duplicated

If a ‘timeout’ occurs or a If a ‘timeout’ occurs or a packet is corrupted, a request packet is corrupted, a request for a resend of that packet is for a resend of that packet is sentsent

After several unsuccessful After several unsuccessful attempts at receiving a attempts at receiving a packet, the connection is packet, the connection is brokenbroken

Network HardwareNetwork Hardware

Network hardware Network hardware includes:includes: Network Interface Network Interface

Controllers (NICs)Controllers (NICs) Switches / HubsSwitches / Hubs RepeatersRepeaters BridgesBridges RoutersRouters Transmission Transmission

media (cabling)media (cabling)

Network interface Network interface controllercontroller

NICs provide a physical NICs provide a physical connection from a connection from a computer (or node) to computer (or node) to the networkthe network

Can be integrated on Can be integrated on the motherboardthe motherboard

Each has a unique 48-Each has a unique 48-bit MAC addressbit MAC address Stored in ROMStored in ROM Media Access ControlMedia Access Control Acts like a name for the Acts like a name for the

cardcard No two the sameNo two the same

http://en.wikipedia.org/wiki/Network_card

HubsHubs

Connects a number Connects a number of NICs togetherof NICs together

Consists of a number Consists of a number of portsof ports

Signals arriving at Signals arriving at one port and sent to one port and sent to all other portsall other ports

Large number of Large number of collisions: a “dumb collisions: a “dumb device”device” http://en.wikipedia.org/wiki/Ethernet_hub

SwitchesSwitches

Sends data only to the Sends data only to the port that requires it – a port that requires it – a “clever hub”“clever hub”

Reduces collisions – and Reduces collisions – and in many cases can in many cases can eliminate them eliminate them altogetheraltogether

Much better than hubs – Much better than hubs – traditionally were much traditionally were much more expensive, but more expensive, but this is no longer the this is no longer the case case

http://en.wikipedia.org/wiki/Network_switch

RepeatersRepeaters

As a network signal As a network signal degrades as it degrades as it travels over large travels over large distances, a distances, a repeater can be repeater can be used to boost the used to boost the signalsignal

Both switches and Both switches and hubs act as hubs act as repeatersrepeaters

http://www.unitedsale.net/products/image-02/D-Link-DWL-G710-Range-Extender-2.jpg

BridgesBridges

Network devices that Network devices that can be used to can be used to connect two different connect two different physical networks physical networks togethertogether

For example, a bridge For example, a bridge could be used to join could be used to join an ethernet network an ethernet network and a fast ethernet and a fast ethernet network or a star and network or a star and a bus topologya bus topology

http://www.stream-shop.ru/Modems/P6212.jpg

RoutersRouters

Acts as a junction between Acts as a junction between two or more networkstwo or more networks

Directs traffic in the same Directs traffic in the same way as a switch, but can also way as a switch, but can also translate and filter (firewall)translate and filter (firewall)

Used to connect LANs to the Used to connect LANs to the InternetInternet For example, a router may For example, a router may

connect to the Internet and be connect to the Internet and be assigned an IP address by an assigned an IP address by an ISPISP

IP addresses may exist within IP addresses may exist within the LAN that duplicate those on the LAN that duplicate those on the Internet, but the router the Internet, but the router ensures these are private to the ensures these are private to the LANLAN

http://shopproducts.howstuffworks.com/Linksys+WRT54GS+Wireless+Router/SF-1/PID-20664068

Bridges vs RoutersBridges vs Routers

Bridges and routers Bridges and routers work in different ways work in different ways (on different OSI layers (on different OSI layers – look up the definition – look up the definition of this if you want to of this if you want to know more)know more)

Bridges are not able to Bridges are not able to convert from one convert from one protocol to another protocol to another where routers canwhere routers can

http://images.amazon.com/images/P/B000H1L5W4.01._AA280_SCLZZZZZZZ_V61775592_.jpg

Network CablingNetwork Cabling

Cables transmit data Cables transmit data from one network node from one network node to anotherto another

Several types of cable Several types of cable exist, each having a exist, each having a particular bandwidthparticular bandwidth Coaxial cableCoaxial cable UTP cableUTP cable Fibre optic cableFibre optic cable

Different types can be Different types can be used within the same used within the same network, but this can network, but this can affect bandwidthaffect bandwidth

Coaxial cableCoaxial cable

Consists of a round Consists of a round conducting wire, surrounded conducting wire, surrounded by an insulating spacer, by an insulating spacer, surrounded by a cylindrical surrounded by a cylindrical conducting sheath conducting sheath

Highly resistant to signal Highly resistant to signal interferenceinterference

Not used much todayNot used much today Bayonet style connectorsBayonet style connectors Often associated with bus Often associated with bus

networksnetworks Bandwidth: 10Mb/sBandwidth: 10Mb/s

http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci211806,00.html

http://www.fotosearch.com/results.asp?keyword=coaxial+cable&category=&searchtype=sss

UTP CableUTP Cable

Unshielded Twisted Pair Unshielded Twisted Pair (UTP) is the most popular (UTP) is the most popular cabling used currentlycabling used currently

Also comes in a Shielded Also comes in a Shielded Twisted Pair version, that Twisted Pair version, that prevent interferenceprevent interference

Cables are twisted Cables are twisted together to decrease together to decrease interferenceinterference

Length limit is 100 metresLength limit is 100 metres CheapCheap

UTP CableUTP Cable

Connector on the end is Connector on the end is called an RJ45called an RJ45

CAT5 is most common CAT5 is most common version and provides a version and provides a bandwidth of 100Mb/sbandwidth of 100Mb/s

CAT5e can provide CAT5e can provide 1000Mb/s (or a 1000Mb/s (or a gigabit/s)gigabit/s)

CAT6a is a future CAT6a is a future specification designed specification designed for 10Gb/s!for 10Gb/s!

http://en.wikipedia.org/wiki/Category_5_cable

http://en.wikipedia.org/wiki/Image:UTP_ethernet_cable_8pin_modular_jack_endings.jpg

Fiber Optic CableFiber Optic Cable Transmits light using the concept of critical Transmits light using the concept of critical

angleangle As a result, cannot bend through tight anglesAs a result, cannot bend through tight angles Not affected by interferenceNot affected by interference Can carry multiple signals at the same timeCan carry multiple signals at the same time Very hand bandwidth possibleVery hand bandwidth possible Expensive and difficult to installExpensive and difficult to install

http://www.fotosearch.com/PDS084/aa031778/

Wireless LANsWireless LANs

Wireless network Wireless network hardware is availablehardware is available Wireless routerWireless router Wireless NICWireless NIC Wireless Access Point Wireless Access Point

(equivalent to a switch)(equivalent to a switch) Relatively cheap and Relatively cheap and

easy to installeasy to install Encryption a must!Encryption a must!

WAP (Wireless Application WAP (Wireless Application Protocol)Protocol)

WPA, WPA2 are betterWPA, WPA2 are better

Wireless LANsWireless LANs

802.11x is a set of standards 802.11x is a set of standards defining the way signals are defining the way signals are formatted, the frequencies they formatted, the frequencies they use and ultimately, transfer use and ultimately, transfer speedspeed 802.11a: (1999), max 54Mb/s802.11a: (1999), max 54Mb/s 802.11b: (1999), max 11Mb/s802.11b: (1999), max 11Mb/s 802.11g: (2003), max 54Mb/s802.11g: (2003), max 54Mb/s 802.11n (draft): (2007), max 802.11n (draft): (2007), max

540Mb/s540Mb/s Typical speeds are often half the Typical speeds are often half the

max speeds due to other network max speeds due to other network traffictraffic

Most devices are backwards Most devices are backwards compatible, for example a device compatible, for example a device might be 802.11bgmight be 802.11bg

http://en.wikipedia.org/wiki/Image:Linksys_Router.png

TopologyTopology

The arrangement of The arrangement of nodes within a networknodes within a network

Main types :Main types : BusBus StarStar Ring*Ring* TreeTree Mesh*Mesh** Not examinable* Not examinable

http://en.wikipedia.org/wiki/Image:NetworkTopologies.png

BusBus

Contains a backbone Contains a backbone which usually runs which usually runs the length of the the length of the networknetwork

Ends are normally Ends are normally terminated using terminated using special connectorsspecial connectors

Often associated with Often associated with coaxial cablecoaxial cable

http://compnetworking.about.com/od/networkdesign/ig/Computer-Network-Topologies/Bus-Network-Topology-Diagram.htm

StarStar In a star network, a In a star network, a

central point is used to central point is used to connect all other connect all other network nodesnetwork nodes

The central point is The central point is usually a hub or a usually a hub or a switchswitch

All messages are routed All messages are routed through the central through the central computer which acts as computer which acts as a traffic cop to prevent a traffic cop to prevent collisions. collisions.

http://compnetworking.about.com/od/networkdesign/ig/Computer-Network-Topologies/Ring-Network-Topology-Diagram.htm

Any connection failure between Any connection failure between a node and the hub will not a node and the hub will not affect the overall system.affect the overall system.

Common network topologyCommon network topology

Easy and cheap to implementEasy and cheap to implement

Tree TopologyTree Topology In a hierarchical network, one In a hierarchical network, one

computer, usually the mainframe, computer, usually the mainframe, controls the entire network. It can be controls the entire network. It can be described as a multiple star network described as a multiple star network with various levels of computers with with various levels of computers with their own smaller network, and so on their own smaller network, and so on down the hierarchical chain. The down the hierarchical chain. The network branches out similar to a tree.network branches out similar to a tree.

A tree topology A tree topology combines star combines star and bus and bus topologiestopologies

Usually this Usually this takes the form takes the form of star of star networks linked networks linked by a network by a network backbone (or a backbone (or a bus network)bus network)

http://compnetworking.about.com/od/networkdesign/ig/Computer-Network-Topologies/Tree-Network-Topology-Diagram.htm

These networks are a lot easier to These networks are a lot easier to repair than other topologies repair than other topologies because you can isolate the problem because you can isolate the problem and it will not have too much of an and it will not have too much of an effect on the rest of the network.effect on the rest of the network.

Server

Switch Switch

Mesh TopologyMesh Topology

In a mesh topology system every In a mesh topology system every computer is connected to every computer is connected to every other computer. Designed to offer other computer. Designed to offer the maximum amount of reliability the maximum amount of reliability for data transit and fault-tolerance.for data transit and fault-tolerance.

The major problem is the amount of The major problem is the amount of cabling necessary to create this cabling necessary to create this topology, plus each link from one topology, plus each link from one device to another requires an device to another requires an individual network card. individual network card.

Not only are physical components Not only are physical components wasted, but the overall capacity to wasted, but the overall capacity to carry data is grossly under-utilised carry data is grossly under-utilised unless all computers are transmitting unless all computers are transmitting to one another almost constantly.to one another almost constantly.

Typically, a mesh network is used Typically, a mesh network is used by governments and the defence by governments and the defence forces, so if one computer or a forces, so if one computer or a number of components are knocked number of components are knocked out by warfare, enough links still out by warfare, enough links still exist for all of the remaining exist for all of the remaining computers to be able to talk to computers to be able to talk to each other.each other.

A mesh topology is one in which paths between network nodes are often changing as the status of nodes changes

The Internet is a perfect example of a mesh topology

http://compnetworking.about.com/od/networkdesign/ig/Computer-Network-Topologies/Mesh-Network-Topology-Diagram.htm

Hybrid TopologyHybrid Topology

A Hybrid topology can be formed by A Hybrid topology can be formed by using elements from more than one using elements from more than one of four topologies - star, ring, bus, of four topologies - star, ring, bus, mesh.mesh.

RingRing All network nodes are All network nodes are

connected in a ringconnected in a ring

A token is passed A token is passed around the network around the network from one computer to from one computer to anotheranother

Only the node that Only the node that holds the token can holds the token can transmit datatransmit data

No collisionsNo collisions

http://compnetworking.about.com/od/networkdesign/ig/Computer-Network-Topologies/Ring-Network-Topology-Diagram.htm

Used to be implemented using Used to be implemented using coaxial cable and is sometimes used coaxial cable and is sometimes used with fibre optic cable todaywith fibre optic cable today

Two rings transmitting data in Two rings transmitting data in different directions can be useddifferent directions can be used

Other Network Other Network Components ContComponents Cont

Network Operating System Network Operating System (NOS)(NOS)

Network Operating Network Operating SystemsSystemsThe most popular NOSes are:

•Novell Netware (better, more expensive at first, cheaper over time)

•Microsoft Server 2003 (now dominant)

They offer similar services.

Network Operating System Network Operating System (NOS)(NOS)

Server is a generic term applied to Server is a generic term applied to any machine running a ‘service’ any machine running a ‘service’ application. Eg access to shared application. Eg access to shared files (file server) or access to files (file server) or access to shared printers (print server).shared printers (print server).

These tasks, and many more, are These tasks, and many more, are controlled by the controlled by the Network Network Operating SystemOperating System..

A NOS operates in the same way to A NOS operates in the same way to the operating system of a PC; the operating system of a PC; however it has special capabilities however it has special capabilities that enable it to connect that enable it to connect computers and peripheral devices, computers and peripheral devices, such as printers, to a LAN.such as printers, to a LAN.

All networking software has a All networking software has a range of functions to perform once range of functions to perform once the connection has been made:the connection has been made:

These functions are:These functions are:

1.1. AdministrationAdministration

2.2. File ManagementFile Management

3.3. Other Resource ManagementOther Resource Management

4.4. SecuritySecurity

1.1. AdministrationAdministration - the - the management of users is the main management of users is the main task. Their authorisation details task. Their authorisation details need to be added, deleted and need to be added, deleted and organised into groups. Backup organised into groups. Backup procedures need to be run.procedures need to be run.

2.2. File ManagementFile Management - the location - the location and transfers of files being used and transfers of files being used needs to be managed between needs to be managed between users and storage locations.users and storage locations.

3.3. Other Resource ManagementOther Resource Management - - such as printer management that such as printer management that involves the spooling of print involves the spooling of print tasks and the allocation of these tasks and the allocation of these tasks to the appropriate printer.tasks to the appropriate printer.

4.4. SecuritySecurity - the access rights of - the access rights of users need to be set up and users need to be set up and monitored.monitored.

Popular Network Popular Network Operating SystemsOperating Systems

UnixUnix - a multi-user, multi-tasking - a multi-user, multi-tasking operating system that can be operating system that can be used on a variety of computers used on a variety of computers ranging from a mainframe to a ranging from a mainframe to a personal computer.personal computer.

Novell NetWareNovell NetWare - the most - the most popular of all NOS. provides users popular of all NOS. provides users with a regular interface that is with a regular interface that is self-sufficient of the hardware self-sufficient of the hardware being used at the time. It is a being used at the time. It is a stand-alone system which is stand-alone system which is situated on the server.situated on the server.

Windows 2000, Windows Server Windows 2000, Windows Server 2003 & Windows Server 20082003 & Windows Server 2008 - is - is the major upgrade of Windows NT; it the major upgrade of Windows NT; it allows for plug-and-play, which allows for plug-and-play, which enables the addition of peripheral enables the addition of peripheral devices. It also uses ‘active directory’ devices. It also uses ‘active directory’ which makes network administration which makes network administration simpler and more stable.simpler and more stable.

LinuxLinux - is a version of Unix that - is a version of Unix that runs on a number of hardware runs on a number of hardware platforms. Because of its stability platforms. Because of its stability it is very popular with Internet it is very popular with Internet Service Providers (ISPs) as an Service Providers (ISPs) as an operating system for web servers.operating system for web servers.

Mac OSXMac OSX ServerServer -is a line of -is a line of computer operating systems computer operating systems developed, marketed, and sold by developed, marketed, and sold by Apple Inc, the latest of which is pre-Apple Inc, the latest of which is pre-loaded on all currently shipping loaded on all currently shipping Macintosh computers. Macintosh computers.

Mac OS X is the successor to the Mac OS X is the successor to the original Mac OS, which had been original Mac OS, which had been Apple's primary operating system Apple's primary operating system since 1984. since 1984.

Unlike its predecessors, Mac OS X Unlike its predecessors, Mac OS X is a Unix-based operating system is a Unix-based operating system built on technology that had been built on technology that had been developed through the second half developed through the second half of the 1980s.of the 1980s.

ServersServers

Robust central computers at the heart of a network.

File servers are the most common server type.

File ServersFile ServersFile servers run the Network operating system (NOS) which handles:

•authenticating users during login

•controlling users’ access to resources based on their rights

•managing print queues

•doing backups

•running centralised software such as virus scanners

•running services like DHCP to give out IP addresses to workstations

• controlling internet services

File ServersFile ServersServers don’t really have anything special in terms of hardware.

Expensive because of their high-quality components, and “scalability” (expandability).

•Memory – servers love lots of RAM.

•Storage –need large and fast hard disks – often RAID (discussed soon)

File Servers vs File Servers vs Desktops 1Desktops 1•CPU Processing power – not very important in a file

server

•Backup – most servers have inbuilt high-capacity tape backup drives to protect against data loss. Tape drives usually use QIC (Quarter Inch Cartridge) DAT (Digital audio tapes) tapes.

Servers are the muscle men in the computer world

File Servers vs File Servers vs Desktops 2Desktops 2Connectivity – servers often have two or more gigabit NICs to increase their data-throughput.

Robustness - servers run all day for years, and need rugged high-quality components

Scalability –the ability to increase the size and power of equipment and networks as required e.g. add 8 hard disks, two power supplies, two NICs, two CPUs, lots of RAM etc.

Designing and engineering this expandability is expensive.

R.A.I.D.R.A.I.D.Redundant Array of Independent [or Inexpensive] Disks) arrays for reliability and/or speed.

RAID uses a group of hard disks that work as a single disk under a RAID controller.

Flavours of RAID: RAID0 to RAID10 (RAID 1 + RAID 0) offer reliability and/or speed (at ever-increasing cost). Includes mirroring (for reliability) and striping (for speed).

RAID disks are usually "Hot Swap".

EXPENSIVE – needs justifying for small organisation

3-disk RAID array

Server farmsServer farmsOn smaller networks, network services are performed by software in a single server.

On busy LANs, multiple servers share the work…

•Login servers – authenticate users•Proxy servers – cache downloads•DHCP servers – allocate IP addresses•Print servers –manage print job queues•Web/FTP servers – serve web pages or files•Email servers –handle email

Proxy ServersProxy Servers

A proxy server is a program on a A proxy server is a program on a special server computer that is special server computer that is used to access websites on behalf used to access websites on behalf of other computers; hence it acts as of other computers; hence it acts as a proxy for the actual Internet.a proxy for the actual Internet.

When a computer wants a website, When a computer wants a website, it is retrieved by the proxy server it is retrieved by the proxy server and then sent to the requesting and then sent to the requesting computer after it has passed the computer after it has passed the firewall program.firewall program.

SecuritySecurity