IPv4 Quick Guide

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IPV4QUICKGUIDE

IPV4OVERVIEW

Thiseraissaidtobetheeraofcomputers.Computershavesignificantlychangedlivesandthewayweusedtolive.Acomputingdevicewhenconnectedtoothercomputingdevice enablesustosharedataandinformationatlightningfastspeed.

WhatisNetwork?

ANetworkintheworldofcomputersissaidtobeacollectionofinterconnectedhosts,viasomesharedmediawhichcanbewiredorwireless.Acomputernetworkenablesitshoststoshareandexchangedataandinformationoverthemedia.NetworkcanbeaLocalAreaNetworkspannedacrossanofficeorMetroAreaNetworkspannedacrossacityorWideAreaNetworkwhichcanbespannedacrosscitiesandprovinces.

ComputernetworkcanbeassimpleastwoPCsconnectedtogetherviaasinglecoppercableoritcanbegrownuptothecomplexitywhereeverycomputerinthisworldisconnectedtoeveryother,theInternet.Anetworkthenincludesmoreandmorecomponentstoreachitsultimategoalofdataexchange.Belowisabriefdescriptionofthecomponentsinvolvedincomputernetwork:

HostsHostsaresaidtobesituatedatultimateendofthenetwork,i.e.ahostisasourceofinformationandanotherhostwillbethedestination.Informationflowsendtoendbetweenhosts.AhostcanbeausersPC,aninternetServer,adatabaseserveretc.

MediaIfwired,thenitcanbecoppercable,fiberopticcable,coaxialcableorifwireless,itcanbefreetoairradiofrequencyorsomespecialwirelessband.Wirelessfrequenciescanbeusedtointerconnectremotesitestoo.

HubAhubisamultiportrepeateranditisusedtoconnecthostsinaLANsegment.Becauseoflowthroughputshubsarenowrarelyused.HubworksonLayer1 ofOSIModel.

SwitchASwitchisamultiportbridgeandisusedtoconnecthostsinaLANsegment.SwitchesaremuchfasterthanHubsandoperateonwirespeed.SwitchworksonLayer2 butLayer3 switchesarealsoavailable.

RouterArouterisLayer3 devicewhichmakesroutingdecisionsforthedata/informationsentforsomeremotedestination.RoutersmakethecoreofanyinterconnectednetworkandtheInternet.

GatewaysAsoftwareorcombinationofsoftwareandhardwareputtingtogetherworksforexchangingdataamongnetworkswhichareusingdifferentprotocolsforsharingdata.

FirewallSoftwareorcombinationofsoftwareandhardware,usedtoprotectusersdatafromunintendedrecipientsonthenetwork/internet.

Allcomponentsinanetworkultimatelyservethehosts.

HostAddressing

Communicationbetweenhostscanhappenonlyiftheycanidentifyeachotheronthenetwork.Inasinglecollisiondomain hostscancommunicatedirectlyviaMACaddress.

MACaddressisafactorycoded48bitshardwareaddresswhichcanalsouniquelyidentifyahostintheworld.Butifahostwantstocommunicatewitharemotehost,i.e.notinthesamesegmentorlogicallynotconnected,thensomemeansofaddressingisrequiredtoidentifytheremotehostuniquely.Alogical

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addressisgiventoallhostsconnectedtoInternetandthislogicaladdressiscalledInternetProtocolAddress.

IPV4OSIMODEL

InternationalStandardOrganizationhasawelldefinedModelforCommunicationSystemsknownasOpenSystemInterconnection,orOSIModel.Thislayeredmodelisaconceptualizedviewofhowonesystemshouldcommunicatewiththeother,usingvariousprotocolsdefinedineachlayer.Further,eachlayerisdesignatedtoawelldefinedpartofcommunicationsystem.Forexample,thePhysicallayerdefinesallthecomponentsofphysicalnature,i.e.wires,frequencies,pulsecodes,voltagetransmissionetc.ofacommunicationsystem.

OSIModelhasfollowingsevenlayers:

ApplicationLayer :Thisiswheretheuserapplicationsitswhoneedstotransferdatabetweenoramonghosts.Forexample:HTTP,filetransferapplication andelectronicmailetc.

PresentationLayer :Thislayerhelpstounderstanddatarepresentationinoneformonahosttootherhostintheirnativerepresentation.Datafromthesenderisconvertedtoonthewiredata andatthereceiversenditisconvertedtothenativerepresentationofthereceiver.

SessionLayer :Thislayerprovidessessionmanagementcapabilitiesbetweenhosts.Forexampleifsomehostneedsapasswordverificationforaccessandifcredentialsareprovidedthenforthatsessionpasswordverificationdoesnothappenagain.Thislayercanassistinsynchronization,dialogcontrolandcriticaloperationmanagement .

TransportLayer :Thislayerprovidesendtoenddatadeliverybetween/amonghosts.ThislayertakesdatafromabovelayerandbreaksitintosmallerunitscalledSegmentsandthengivesittoNetworklayerfortransmission.

NetworkLayer :Thislayerhelpstouniquelyidentifyhostsbeyondthesubnetsanddefinesthepathwhichthepacketswillfolloworberoutedtoreachthedestination.

DataLinkLayer :Thislayertakestherawtransmissiondata from

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PhysicalLayerandmakesDataFramesandsendsthattoupperlayerandviceversa.Thislayeralsochecksanytransmissionerrorsandsortitoutaccordingly.

PhysicalLayer :Thislayerdealswithhardwaretechnologyandactualcommunicationmechanismlikesignaling,voltage,cabletypeandlengthetc.

NetworkLayer

Thenetworklayerisresponsibleforcarryingdatafromonehosttoanother.Itprovidesmeanstoallocatelogicaladdressestohostsandidentifythemuniquelyusingthesame.NetworklayertakesdataunitsfromTransportLayerandcutsthemintosmallerunitcalledDataPacket.

Networklayerdefinesthedatapath,thepacketsshouldfollowtoreachthedestination.Routersworkonthislayerandprovidesmechanismtoroutedatatoitsdestination.

IPV4TCP/IPMODEL

MajorlyoftheinternetusesaprotocolsuitecalledtheInternetProtocolSuitealsoknownasTCP/IPprotocolsuite.Thissuiteisacombinationofprotocolswhichencompassesanumberofdifferentprotocolsfordifferentpurposeandneed.BecausethetwomajorprotocolsinthissuitesareTCP

andIP ,thisiscommonlytermedasTCP/IPProtocolsuite.Thisprotocolsuitehasitsownreferencemodelwhichitfollowsovertheinternet.IncontrastwithOSImodel,thismodelofprotocolscontainslesslayers.

[ComparativedepictionofOSIandTCP/IPReferenceModels]

Thismodelisindifferenttotheactualhardwareimplementation,i.e.PhysicallayerofOSIModel.Thisiswhythismodelcanbeimplementedonalmostallunderlyingtechnologies.TransportandInternetlayerscorrespondstothesamepeerlayers.AllthreetoplayersofOSIModelarecompressedtogetherinsingleApplicationlayerofTCP/IPModel.

InternetProtocolVersion4

InternetProtocolisoneofthemajorprotocolinTCP/IPprotocolssuite.ThisprotocolworksatNetworklayerofOSImodelandatInternetlayerofTCP/IPmodel.Thusthisprotocolhastheresponsibilityof

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identificationofhostsbasedupontheirlogicaladdressesandtoroutedatabetween/amongthemovertheunderlyingnetwork.

IPprovidesamechanismtouniquelyidentifyhostbyIPaddressingscheme.IPusesbesteffortdelivery,i.e.itdoesnotguaranteethatpacketswouldbedeliveredtodestinedhostbutitwilldoitsbesttoreachthedestination.InternetProtocolversion4uses32bitlogicaladdress.

IPV4PACKETSTRUCTURE

InternetProtocolbeingalayer3protocol takesdataSegmentsfromlayer4 anddividesitintowhatscalledpacket.IPpacketencapsulatesdataunitreceivedfromabovelayerandaddsitsownheaderinformation.

TheencapsulateddataisreferredtoasIPPayload.IPheadercontainsallthenecessaryinformationtodeliverthepacketattheotherend.

IPheaderincludesmanyrelevantinformationincludingVersionNumber,which,inthiscontext,is4.Otherdetailsareasfollows:

Version:Versionno.ofInternetProtocolused

IHL:InternetHeaderLength,LengthofentireIPheader

DSCP:DifferentiatedServicesCodePoint,ThisisTypeofService.

ECN:ExplicitCongestionNotification,carriesinformationaboutthecongestionseenintheroute.

TotalLength:LengthofentireIPPacket

Identification:IfIPpacketisfragmentedduringthetransmission,allthefragmentscontainsameidentificationno.toidentifyoriginalIPpackettheybelongto.

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Flags:Asrequiredbythenetworkresources,ifIPPacketistoolargetohandletheseflagstellthatiftheycanbefragmentedornot.Inthis3bitflag,theMSBisalwayssetto0.

FragmentOffset:ThisoffsettellstheexactpositionofthefragmentintheoriginalIPPacket.

TimetoLive:Toavoidloopinginthenetwork,everypacketissentwithsomeTTLvalueset,whichtellsthenetworkhowmanyrouters thispacketcancross.Ateachhop,itsvalueisdecrementedbyoneandwhenthevaluereacheszero,thepacketisdiscarded.

Protocol:TellstheNetworklayeratthedestinationhost,towhichProtocolthispacketbelongsto,i.e.thenextlevelProtocol.ForexampleprotocolnumberofICMPis1,TCPis6andUDPis17.

HeaderChecksum:Thisfieldisusedtokeepchecksumvalueofentireheaderwhichisthenusedtocheckifthepacketisreceivederrorfree.

SourceAddress:32bitaddressoftheSender ofthepacket.

DestinationAddress:32bitaddressoftheReceiver ofthepacket.

Options:Thisisoptionalfield,whichisusedifthevalueofIHLisgreaterthan5.TheseoptionmaycontainvaluesforoptionssuchasSecurity,RecordRoute,TimeStampetc.

IPV4ADDRESSING

IPv4supportsthreedifferenttypeofaddressingmodes:

UnicastAddressingMode:

Inthismode,dataissentonlytoonedestinedhost.TheDestinationAddressfieldcontains32bitIPaddressofthedestinationhost.Hereclientsendsdatatothetargetedserver:

BroadcastAddressingMode:

Inthismodethepacketisaddressedtoallhostsinanetworksegment.TheDestinationAddressfieldcontainsspecialbroadcastaddressi.e.255.255.255.255.Whenahostseesthispacketonthenetwork,itisboundtoprocessit.Hereclientsendspacket,whichisentertainedbyalltheServers:

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MulticastAddressingMode:

Thismodeisamixofprevioustwomodes,i.e.thepacketsentisneitherdestinedtoasinglehostnorallthehostonthesegment.Inthispacket,theDestinationAddresscontainsspecialaddresswhichstartswith224.x.x.xandcanbeentertainedbymorethanonehost.

HereaserversendspacketswhichisentertainedbymorethanoneServers.EverynetworkhasoneIPaddressreservedfornetworknumberwhichrepresentsthenetworkandoneIPaddressreservedforBroadcastAddress,whichrepresentsallthehostinthatnetwork.

HierarchicalAddressingScheme

IPv4useshierarchicaladdressingscheme.AnIPaddresswhichis32bitsinlength,isdividedintotwoorthreepartsasdepicted:



AsingleIPaddresscancontaininformationaboutthenetworkanditssubnetworkandultimatelythehost.ThisschemeenablesIPAddresstobehierarchicalwhereanetworkcanhavemanysubnetworkswhichinturncanhavemanyhosts.

SubnetMask

The32bitIPaddresscontainsinformationaboutthehostanditsnetwork.Itisverynecessarytodistinguishtheboth.Forthis,routersuseSubnetMask,whichisaslongasthesizeofthenetworkaddressintheIPaddress.SubnetMaskisalso32bitslong.IftheIPaddressinbinaryisANDedwithitsSubnetMask,theresultyieldstheNetworkaddress.Forexample,saytheIPAddress192.168.1.152andtheSubnetMaskis255.255.255.0then

ThiswaySubnetMasthelpsextractNetworkIDandHostfromanIPAddress.Itcanbeidentifiednowthat192.168.1.0istheNetworknumberand192.168.1.152isthehostonthatnetwork.

BinaryRepresentation

Thepositionalvaluemethodisthesimplestformofconvertingbinaryfromdecimalvalue.IPaddressis32bitvaluewhichisdividedinto4octets.Abinaryoctetcontains8bitsandthevalueofeachbitcanbedeterminedbythepositionofbitvalue'1'intheoctet.

Positionalvalueofbitsisdeterminedby2raisedtopower ,thatisthevalueofabit1atposition6is261thatis25thatis32.Thetotalvalueoftheoctetisdeterminedbyaddingupthepositionalvalueofbits.Thevalueof11000000is128+64=192.SomeExamplesareshowninthetablebelow:

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IPV4ADDRESSCLASSES

InternetProtocolhierarchycontainsseveralclassesofIPAddressestobeusedefficientlyinvarioussituationaspertherequirementofhostspernetwork.Broadly,IPv4Addressingsystemisdividedinto5classesofIPAddresses.Allthe5classesareidentifiedbythefirstoctetofIPAddress.

InternetCorporationforAssignedNamesandNumbersresponsibleforassigningIPaddresses.

Thefirstoctetreferredhereistheleftmostofall.TheoctetsnumberedasfollowsdepictingdotteddecimalnotationofIPAddress:



Numberofnetworksandnumberofhostsperclasscanbederivedbythisformula:

WhencalculatinghostsIPaddresses,2IPaddressesaredecreasedbecausetheycannotbeassignedtohostsi.e.thefirstIPofanetworkisnetworknumberandthelastIPisreservedforBroadcastIP.

ClassAAddress

Thefirstbitofthefirstoctetisalwayssetto0 .Thusthefirstoctetrangesfrom1127,i.e.

ClassAaddressesonlyincludeIPstartingfrom1.x.x.xto126.x.x.xonly.TheIPrange127.x.x.xisreservedforloopbackIPaddresses.

ThedefaultsubnetmaskforClassAIPaddressis255.0.0.0whichimpliesthatClassAaddressingcanhave126networks(272)and16777214hosts(2242).

ClassAIPaddressformatthus,is0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

ClassBAddress

AnIPaddresswhichbelongstoclassBhasthefirsttwobitsinthefirstoctetsetto10,i.e.

ClassBIPAddressesrangefrom128.0.x.xto191.255.x.x.ThedefaultsubnetmaskforClassBis255.255.x.x.

ClassBhas16384(214)Networkaddressesand65534(2162)Hostaddresses.

ClassBIPaddressformatis,10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

ClassCAddress

ThefirstoctetofClassCIPaddresshasitsfirst3bitssetto110,thatis

ClassCIPaddressesrangefrom192.0.0.xto192.255.255.x.ThedefaultsubnetmaskforClassBis

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255.255.255.x.

ClassCgives2097152(221)Networkaddressesand254(282)Hostaddresses.

ClassCIPaddressformatis110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

ClassDAddress

VeryfirstfourbitsofthefirstoctetinClassDIPaddressesaresetto1110,givingarangeof

ClassDhasIPaddressragefrom224.0.0.0to239.255.255.255.ClassDisreservedforMulticasting.Inmulticastingdataisnotdestinedforaparticularhost,that'swhythereisnoneedtoextracthostaddressfromtheIPaddress,andClassDdoesnothaveanysubnetmask.

ClassEAddress

ThisIPClassisreservedforexperimentalpurposesonlylikeforR&DorStudy.IPaddressesinthisclassrangesfrom240.0.0.0to255.255.255.254.LikeClassD,thisclasstooisnotequippedwithanysubnetmask.

IPV4SUBNETTING

EachIPclassisequippedwithitsowndefaultsubnetmaskwhichboundsthatIPclasstohaveprefixednumberofNetworksandprefixednumberofHostspernetwork.ClassfulIPaddressingdoesnotprovideanyflexibilityofhavinglessnumberofHostsperNetworkormoreNetworksperIPClass.

CIDRorClasslessInterDomainRoutingprovidestheflexibilityofborrowingbitsofHostpartoftheIPaddressandusingthemasNetworkinNetwork,calledSubnet.Byusingsubnetting,onesingleClassAIPaddressescanbeusedtohavesmallersubnetworkswhichprovidesbetternetworkmanagementcapabilities.

ClassASubnets

InClassA,onlythefirstoctetisusedasNetworkidentifierandrestofthreeoctetsareusedtobeassignedtoHosts .TomakemoresubnetinClassA,bitsfromHostpartareborrowedandthesubnetmaskischangedaccordingly.

Forexample,ifoneMSB isborrowedfromhostbitsofsecondoctetandaddedtoNetworkaddress,itcreatestwoSubnets(21=2)with(2232)8388606HostsperSubnet.

TheSubnetmaskischangedaccordinglytoreflectsubnetting.GivenbelowisalistofallpossiblecombinationofClassAsubnets:

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Incaseofsubnettingtoo,theveryfirstandlastIPaddressofeverysubnetisusedforSubnetNumberandSubnetBroadcastIPaddressrespectively.BecausethesetwoIPaddressescannotbeassignedtohosts,Subnettingcannotbeimplementedbyusingmorethan30bitsasNetworkBitswhichprovideslessthantwohostspersubnet.

ClassBSubnets

ByDefault,usingClassfulNetworking,14bitsareusedasNetworkbitsproviding(214)16384Networksand(2161)65534Hosts.ClassBIPAddressescanbesubnettedthesamewayasClassAaddresses,byborrowingbitsfromHostbits.BelowisgivenallpossiblecombinationofClassBsubnetting:



ClassCSubnets

ClassCIPaddressesnormallyassignedtoaverysmallsizenetworkbecauseitonlycanhave254hostsinanetwork.GivenbelowisalistofallpossiblecombinationofsubnettedClassBIPaddress:

IPV4VARIABLELENGTHSUBNETMASKING

InternetServiceProvidersmayfaceasituationwheretheyneedtoallocateIPsubnetsofdifferentsizesaspertherequirementofcustomer.OnecustomermayaskClassCsubnetof3IPaddressesandanothermayaskfor10IPs.ForanISP,itisnotfeasibletodividetheIPaddressesintofixedsizesubnets,ratherhemaywanttosubnetthesubnetsinsuchawaywhichresultsinminimumwastageofIPaddresses.

Forexample,anadministratorhave192.168.1.0/24network.Thesuffix/24tellsthenumberofbitsusedfornetworkaddress.Heishavingthreedifferentdepartmentswithdifferentnumberofhosts.Salesdepartmenthas100computers,Purchasedepartmenthas50computers,Accountshas25computersandManagementhas5computers.InCIDR,thesubnetsareoffixedsize.Usingthesamemethodologytheadministratorcannotfulfillalltherequirementsofthenetwork.

ThefollowingprocedureshowshowVLSMcanbeusedinordertoallocatedepartmentwiseIPaddressesasmentionedintheexample.

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Step1

MakealistofSubnetspossible.

Step2

SorttherequirementsofIPsindescendingorder .

Sales100

Purchase50

Accounts25

Management5

Step3

AllocatethehighestrangeofIPstothehighestrequirement,solet'sassign192.168.1.0/25toSalesdepartment.ThisIPsubnetwithNetworknumber192.168.1.0has126validHost

IPaddresseswhichsatisfytherequirementofSalesDepartment.ThesubnetMaskusedforthissubnethas10000000asthelastoctet.

Step4

Allocatethenexthighestrange,solet'sassign192.168.1.128/26 toPurchasedepartment.ThisIPsubnetwithNetworknumber192.168.1.128has62validHostIPAddresseswhichcanbeeasilyassignedtoallPurchasedepartment'sPCs.Thesubnetmaskusedhas11000000inthelastoctet.

Step5

Allocatethenexthighestrange,i.e.Accounts.Therequirementof25IPscanbefulfilledwith192.168.1.192/27 IPsubnet,whichcontains30validhostIPs.ThenetworknumberofAccountsdepartmentwillbe192.168.1.192.Thelastoctetofsubnetmaskis11100000.

Step6

AllocatenexthighestrangetoManagement.TheManagementdepartmentcontainsonly5computers.Thesubnet192.168.1.224/29withMask255.255.255.248hasexactly6validhostIPaddresses.SothiscanbeassignedtoManagement.Thelastoctetofsubnetmaskwillcontain11111000.

ByusingVLSM,theadministratorcansubnettheIPsubnetsuchawaythatleastnumberofIPaddressesarewasted.EvenafterassigningIPstoeverydepartment,theadministrator,inthisexample,stillleftwithplentyofIPaddresseswhichwasnotpossibleifhehasusedCIDR.

IPV4RESERVEDADDRESSES

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TherearefewReservedIPv4addressspaceswhichcannotbeusedontheinternet.TheseaddressesservespecialpurposeandcannotberoutedoutsideLocalAreaNetwork.

PrivateIPAddresses

EveryclassofIP, A,B&C hassomeaddressesreservedasPrivateIPaddresses.TheseIPscanbeusedwithinanetwork,campus,companyandareprivatetoit.TheseaddressescannotberoutedonInternetsopacketscontainingtheseprivateaddressesaredroppedbytheRouters.

Inordertocommunicatewithoutsideworld,Internet,theseIPaddressesmusthavetobetranslatedtosomepublicIPaddressesusingNATprocessorWebProxyservercanbeused.

ThesolepurposetocreateseparaterangeofprivateaddressesistocontrolassignmentofalreadylimitedIPv4addresspool.ByusingprivateaddressrangewithinLAN,therequirementofIPv4addresseshasgloballydecreasedsignificantly.IthasalsohelpeddelayingtheIPv4addressexhaustion.

IPclass,whileusingprivateaddressrange,canbechosenasperthesizeandrequirementoftheorganization.LargerorganizationmaychooseclassAprivateIPaddressrangewheresmallermayoptforclassC.TheseIPaddressescanbefurthersubnettedbeassignedtodepartmentswithinanorganization.

LoopbackIPAddresses

TheIPaddressrange127.0.0.0127.255.255.255isreservedforloopbacki.e.aHostsselfaddress.Alsoknownaslocalhostaddress.ThisloopbackIPaddressismanagedentirelybyandwithintheoperatingsystem.Usingloopbackaddresses,enabletheServerandClientprocessesonasinglesystemtocommunicatewitheachother.Whenaprocesscreatesapacketwithdestinationaddressasloopbackaddress,theoperatingsystemloopsitbacktoitselfwithouthavinganyinterferenceofNIC.

Datasentonloopbackisforwardbytheoperatingsystemtoavirtualnetworkinterfacewithinoperatingsystem.Thisaddressismostlyusedfortestingpurposeslikeclientserverarchitectureonasinglemachine.Otherthanthat,ifahostmachinecansuccessfullyping127.0.0.1oranyIPfromloopbackrange,impliesthattheTCP/IPsoftwarestackonthemachineissuccessfullyloadedandworking.

LinklocalAddresses

IncaseoftheHostisnotabletoacquireanIPaddressfromDHCPserverandithasnotbeenassignedanyIPaddressmanually,thehostcanassignitselfanIPaddressfromarangeofreservedLinklocaladdresses.Linklocaladdressrangeis169.254.0.0169.254.255.255.

AssumeanetworksegmentwhereallsystemsareconfiguredtoacquireIPaddressesfromaDHCPserverconnectedtothesamenetworksegment.IftheDHCPserverisnotavailable,nohostonthesegmentwillbeabletocommunicatetoanyother.Windows ,andMacOS supportthisfunctionalityofselfconfigurationofLinklocalIPaddress.InabsenceofDHCPserver,everyhostmachinerandomlychoosesanIPaddressfromtheabovementionedrangeandthencheckstoascertainbymeansofARP,ifsomeotherhostalsohasnotconfiguredhimselfwiththesameIPaddress.Onceallhostareusinglinklocaladdressesofsamerange,theycancommunicatetoeachother.

TheseIPaddressescannothelpsystemtocommunicatewhentheydonotbelongtothesamephysicalorlogicalsegment.TheseIPsarealsonotroutable.

IPV4EXAMPLE

ThissectiontellshowactualcommunicationhappensontheNetworkusingInternetProtocolversion4.

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Packetflowinnetwork

AllthehostsinIPv4environmentareassigneduniquelogicalIPaddresses.Whenahostwantstosendsomedatatoanotherhostonthenetwork,itneedsthephysical addressofthedestinationhost.TogettheMACaddress,thehostbroadcastsARPmessageandaskstogivetheMACaddresswhoeveristheownerofdestinationIPaddress.AllthehostonthatsegmentreceivesARPpacketbutonlythehostwhichhasitsIPmatchingwiththeoneinARPmessage,replieswithitsMACaddress.OncethesenderreceivestheMACaddressofreceivingstation,dataissentonthephysicalmedia.

Incase,theIPdoesnotbelongtothelocalsubnet.ThedataissenttothedestinationbymeansofGatewayofthesubnet.Tounderstandthepacketflowwemustfirstunderstandfollowingcomponents:

MACAddress:MediaAccessControlAddressis48bitfactoryhardcodedphysicaladdressofnetworkdevicewhichcanuniquelybeidentified.Thisaddressisassignedbydevicemanufacturers.

AddressResolutionProtocol:AddressResolutionProtocolisusedtoacquiretheMACaddressofahostwhoseIPaddressisknown.ARPisaBroadcastpacketwhichisreceivedbyallthehostinthenetworksegment.ButonlythehostwhoseIPismentionedinARPrespondstoitprovidingitsMACaddress.

ProxyServer:ToaccessInternet,networksusesProxyServerwhichhasapublicIPassigned.AllPCsrequestProxyServerforaServeronInternet,ProxyServeronbehalfofPCsendstherequesttoserverandwhenitreceivesresponsefromtheServer,theProxyServerforwardsittotheclientPC.ThisisawaytocontrolInternetaccessincomputernetworksandithelpstoimplementwebbasedpolicies.

DynamicHostControlProtocol:DHCPisaservicebywhichahostisassignedIPaddressfromapredefinedaddresspool.DHCPserveralsoprovidesnecessaryinformationsuchasGatewayIP,DNSServerAddress,leaseassignedwiththeIPetc.ByusingDHCPservicesnetworkadministratorcanmanageassignmentofIPaddressesatease.

DomainNameSystem:ThisisverylikelythatauserdoesnotknowtheIPaddressofaremoteServerhewantstoconnectto.Butheknowsthenameassignedtoitforexample,tutorialpoints.com.WhentheusertypesinthenameofremoteserverhewantstoconnecttothelocalhostbehindthescreenssendsaDNSquery.DomainNameSystemisamethodtoacquiretheIPaddressofthehostwhoseDomainNameisknown.

NetworkAddressTranslation:AlmostallPCsinacomputernetworkareassignedprivateIPaddresseswhicharenotroutableonInternet.AssoonasarouterreceivesanIPpacketwithprivateIPaddressitdropsit.InordertoaccessServersonpublicprivateaddress,computernetworksuseanaddresstranslationservice,whichtranslatesbetweenpublicandprivateaddresses,calledNetworkAddressTranslation.WhenaPCsendsanIPpacketoutofaprivatenetwork,NATchangestheprivateIPaddresswithpublicIPaddressandviceversa.

Wecannowdescribethepacketflow.Assumethatauserwantstoaccesswww.TutorialsPoint.comfromherpersonalcomputer.SheishavinginternetconnectionfromherISP.Thefollowingstepswillbetakenbythesystemtohelpherreachdestinationwebsite.

Step:1AcquiringanIPAddress

WhenusersPCbootsup,itsearchesforaDHCPservertoacquireanIPaddress.Forthesame,PCsendsaDHCPDISCOVERbroadcastwhichisreceivedbyoneormoreDHCPserversonthesubnetandtheyallrespondwithDHCPOFFERwhichincludesallthenecessarydetailslikeIP,subnet,Gateway,DNSetc.PCsendsDHCPREQUESTpacketinordertorequesttheofferedIPaddress.Finally,DHCPsendsDHCPACKpackettotellPCthatitcankeeptheIPforsomegivenamountoftimeakaIPlease.

AlternativelyaPCcanbeassignedanIPaddressmanuallywithouttakinganyhelpfromDHCPServer.WhenaPCiswellconfiguredwithIPaddressdetails,itcannowspeaktoothercomputersallovertheIPenablednetwork.

Step:2DNSquery



Whenauseropensawebbrowserandtypeswww.tutorialpoints.comwhichisadomainnameandaPCdoesnotunderstandhowtocommunicatewiththeserverusingdomainnames.PCsendsaDNSqueryoutonthenetworkinordertoobtaintheIPaddresspertainingtothedomainname.ThepreconfiguredDNSserverrespondsthequerywithIPaddressofthedomainnamespecified.

Step:3ARPrequest

ThePCfindsthatthedestinationIPaddressdoesnotbelongtohisownIPaddressrangeandithastoforwardtherequesttotheGateway.GatewayinthisscenariocanbearouteroraProxyServer.ThoughGatewaysIPaddressisknowntotheclientmachinebutcomputersdonotexchangedataonIPaddressesrathertheyneedmachineshardwareaddresswhichisLayer2factorycodedMACaddress.ToobtaintheMACaddressoftheGatewaytheclientPCbroadcastsanARPrequestsaying"WhoownsthisIPaddress?"TheGatewayinresponsetotheARPquerysendsitMACaddress.UponreceivingMACaddressPCsendsthepacketstoGateway.

AnIPpackethasbothsourceanddestinationaddressesandthisconnectshostwitharemotehostlogically.WhereasMACaddresseshelpssystemsonasinglenetworksegmenttotransferactualdata.ThisisimportantthatsourceanddestinationMACaddresseschangeastheytravelacrosstheInternet

butsourceanddestinationIPaddressneverchanges.

IPV4SUMMARY

TheInternetProtocolversion4wasdesignedtobeallocatedtoapprox.4.3billionaddresses.AtthebeginningofInternetthiswasconsideredamuchwideraddressspaceforwhichtherewasnothingtoworryabout.

ThesuddengrowthinInternetusersanditswidespreadusehasexponentiallyincreasethenumberofdeviceswhichneedsrealanduniqueIPtobeabletocommunicate.Gradually,anIPisrequiredbyalmosteverydigitalequipmentwhichweremadetoeasehumanlife,suchasMobilePhones,Carsandotherelectronicdevices.Thenumberofdevices expandedthedemandforextraIPaddresses,whichwerenotconsideredearlier.

AllocationofIPv4isgloballymanagedbyInternetAssignedNumbersAuthority undercoordinationwithInternetCorporationforAssignedNamesandNumbers .IANAworkscloselywithRegionalInternetRegistries,whichinturnsareresponsibleforefficientlydistributeIPaddressintheirterritories.TherearefivesuchRIRexist.AccordingtoIANAreports,alltheIPv4addressblockshavebeenallocated.Tocopeupwiththesituation,asanearlystepsthefollowingpracticeswerebeingdone:

PrivateIPs:FewblocksofIPsweredeclaredforprivateusewithinaLANsothattherequirementforpublicIPaddressescanbereduced.

NAT:NetworkaddresstranslationisamechanismbywhichmultiplePCs/hostswithprivateIPaddressesareenabledtoaccessusingoneorfewpublicIPaddresses.

UnusedPublicIPswerereclaimedbyRIRs.

InternetProtocolv6

IETF hasredesignedIPaddressesandtomitigatethedrawbacksofIPv4.ThenewIPaddresshasversion6andis128bitaddress,bywhicheverysingleinchoftheearthcanbegivenmillionsofIPaddresses.

TodaymajorityofdevicesrunningonInternetareusingIPv4anditisnotpossibletoshiftthemtoIPv6incomingdays.TherearemechanismprovidedbyIPv6,bywhichIPv4andIPv6cancoexistunlesstheInternetentirelyshiftstoIPv6:

DualIPStack

Tunneling

NATProtocolTranslation

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Documents

IPv4 Quick Guide