第4章：OSI层次：网络层

OSILayer3:

NetworkLayerRoutingandAddressingLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPLayer3ResponsibilitiesMovedatathroughnetworksUseahierarchicaladdressingscheme(opposedtoMACaddressing,whichisflat)SegmentnetworkandcontrolflowoftrafficReducecongestionTalktoothernetworksLayer3DevicesRoutersInterconnectnetworksegmentsornetworksMakelogicaldecisionsbasedonIPaddressesDeterminebestpathSwitchpacketsfromincomingportstooutgoingportsLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPLayer3Packet/DatagramHeaderincludessourceANDdestinationaddresses固定部分可变部分04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特数据部分首部传送IP数据报首部可变部分首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特数据部分首部传送IP数据报固定部分首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特数据部分首部传送IP数据报固定部分可变部分首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分版本——占4bit，指IP协议的版本目前的IP协议版本号为4(即IPv4)首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分首部长度——占4bit，可表示的最大数值是15个单位(一个单位为4字节)因此IP的首部长度的最大值是60字节。首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分服务类型——占8bit，用来获得更好的服务这个字段以前一直没有被人们使用首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分总长度——占16bit，指首部和数据之和的长度，单位为字节，因此数据报的最大长度为65535字节。总长度必须不超过最大传送单元MTU。

首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分标识(identification)占16bit，它是一个计数器，用来产生数据报的标识。首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分标志占3bit，最高位为0MF为0表示最后一个分片首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分片偏移(12bit)指出：较长的分组在分片后某片在原分组中的相对位置。片偏移以8个字节为偏移单位。偏移=0/8=0偏移=0/8=0偏移=1400/8=175偏移=2800/8=350140028003799279913993799需分片的数据报数据报片1首部数据部分共3800字节首部1首部2首部3字节0数据报片2数据报片314002800字节0IP数据报分片的举例首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分生存时间(8bit)记为TTL(TimeToLive)数据报在网络中可通过的路由器数的最大值。首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分协议(8bit)字段指出此数据报携带的数据使用何种协议以便目的主机的IP层将数据部分上交给哪个处理过程运输层网络层首部TCPUDPICMPIGMPOSPF数据部分IP数据报协议字段指出应将数据部分交给哪一个进程首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分首部检验和(16bit)字段只检验数据报的首部不包括数据部分。这里不采用

CRC

检验码而采用简单的计算方法。发送端接收端16bit字116bit字2置为全0检验和16bit字n16bit反码算术运算求和……取反码数据报首部IP数据报16bit检验和16bit字116bit字216bit检验和16bit字n16bit反码算术运算求和16bit结果……取反码数据部分若结果为0,则保留；否则，丢弃该数据报数据部分不参与检验和的计算首部04816192431版本标志生存时间协议标识服务类型总长度片偏移填充首部检验和源地址目的地址可选字段（长度可变）比特首部长度01234567DTRC未用优先级数据部分比特固定部分可变部分源地址和目的地址都各占4字节NetworkLayerAddressesIPaddressesare32bitslongTheyarerepresentedasfouroctetsindotteddecimalformatTheIPaddresshastwocomponents:ThenetworkIDThehostIDLayer3AddressesNetworkIDassignedbyARIN(AmericanRegistryforInternetNumbers,)identifiesthenetworktowhichadeviceisattachedmaybeidentifiedbyone,two,orthreeofthefirstthreeoctetsHostIDassignedbyanetworkadministratoridentifiesthespecificdeviceonthatnetworkmaybeidentifiedbyone,two,orthreeofthelastthreeoctetsNetworkHostIPAddressesDifferentclassaddressesreservedifferentamountsofbitsforthenetworkandhostportionsoftheaddressClassANHHHClassBNNHHClassCNNNH0Network#Host#8810Network#Host#88110Network#Host#0–127ClassAaddress128-191ClassBaddress192–223ClassCaddress224–239ClassD–Multicast240–255ClassE-ResearchClassAClassBClassCClassesNumberofHostsThemaximumnumberofhostsvaryforeachclass.ClassAhas16,777,214availablehosts(224

–2)ClassBhas65,534availablehosts(216

–2)ClassChas254availablehosts(28

–2)ThefirstaddressineachnetworkisreservedforthenetworkaddressThelastaddressisreservedforthebroadcastaddress.ReservedAddressesNetworkAddressAnIPaddressthatendswithbinary0sinthehostpartoftheaddressClassAnetworkaddressexample:HostsonanetworkcanonlycommunicatedirectlywithotherhostsiftheyhavethesamenetworkID.ReservedAddressesBroadcastAddressisusedtosenddatatoallofthedevicesonanetwork.BroadcastIPaddressesendwithbinary1sinthehostpartoftheaddress.ClassBbroadcastaddressexample:55

(decimal255=binary11111111)ClassA

:areservednetworknumber55:abroadcastnumber

ClassB:areservednetwork

number55:abroadcast

numberClassC:areservednetwork

number55:abroadcast

numberIPAddressingTherearecertainIPaddressrangesreservedforprivateIPaddressingschemes.IPaddressdepletionanditssolutions:NATCIDRIPv6PrivateAddressSpace-55-55-55Networkadministratorssometimesneedtodividenetworksintosmallernetworks,calledsubnets,inordertoprovideextraflexibilityBitsareborrowedfromthehostfieldandaredesignatedasthesubnetfieldSubnetNetworkSubnetHostNetworkHostBasicsofSubnettingSubnetsaresmallerdivisionsofnetworksprovideaddressingflexibility.Subnetaddressesareassignedlocally,usuallybyanetworkadministrator.Subnetsreduceabroadcastdomain.HowmanybitscanIborrow?SizeofHostFieldMaximum#ofborrowedbitsClassA2422ClassB1614ClassC86TheminimumnumberofbitsyoucanborrowistwoTheminimumnumberofbitsborrowedis2,WHY?Ifyouweretoborrowonly1bit,tocreateasubnet,thenyouwouldonlyhaveanetworknumber-the.0network-andthebroadcastnumber-the.1networkThemaximumnumberofbitsthatcanbeborrowedcanbeanynumberthatleavesatleast2bits,remaining,forthehostnumberHowmanybitscanIborrow?Wemuststrikeabalancebetweenthenumberofsubnetsrequired,thehostspersubnetthatisacceptable,andtheresultingwasteofaddresses.NumberofBitsBorrowedNumberofSubnetsCreatedNumberofHostsPerSubnetTotalNumberofHostsPercentUsed226212449%363018071%4141419677%530618071%662212449%Byproduct:WasteAddressesClassCSubnetmaskAlias:extendednetworkprefixdefinehowmanybitsweusetoconstructthenetwork,andhowmanybitstodescribethehostaddressesSubnetMaskClassA ClassB ClassC CalculatingaSubnetWewillsubnettheIPaddress:Need:13subnets10hostsoneachsubnetStep#1DeterminethedefaultsubnetmaskWhatclassIPaddressisthis?ClassCClassCdefaultsubnetmask:Step#2FiguretheactualnumberofsubnetsandhostsbyborrowingbitsfromhostIDLet’sseehowmanysubnetsandhosts13subnets10hostsoneachsubnetBorrow4bitsfromthehostStep#3XXXXHHHH16possiblesubnets16possiblehostsforeachsubnetStep#3continued…Weget16possiblesubnetsand16possiblehostsforeachsubnetbecause:Forthe4bitsborrowedeachbitcanbea1ora0leavingyouwith24or16possiblecombinations.Thesamegoesforthe4leftoverhostbits.Important:Thereareonly14availablesubnetsandhostsoneachsubnet.Why?Step#3continued…Becauseyoucannotusethefirstandlastsubnet.Becauseyoucannotusethefirstandlastaddresswithineachsubnet.Foreach,oneisthebroadcastaddressandoneisthenetworkaddress.Step#4Determinethesubnetmask.XXXXHHHHWhereXrepresentstheborrowedbitsforsubnetting.Step#4continued…AddtheplacevaluesofXtogethertogetthelastoctetdecimalvalueofthesubnetmask.128+64+32+16=240Thesubnetmaskis:40ThesubnetmaskisusedtorevealthesubnetandhostaddressfieldsinIPaddresses.Step5DeterminetherangesofhostaddressesSubnet#SubnetBitsHostBitsInDecimal100000000-1111.0-1111.16-.31300100000-1111.32-.47400110000-1111.48-.63501000000-1111.64-.79601010000-1111.80-.95701100000-1111.96-.111801110000-1111.112-.127Step5continued…Subnet#SubnetBitsHostBitsInDecimal910000000-1111.128-.1431010010000-1111.144-.1591110100000-1111.160-.1751210110000-1111.176-.1911311000000-1111.192-.2071411010000-1111.208-.2231511100000-1111.224-.2391611110000-1111.240-.255Step5continued…16possiblesubnets.16possiblehostsoneachsubnet14availablesubnets14availablehostsoneachsubnetFiguringSubnetNetworkAddressesStep#1:ChangetheIPhostaddresstobinary.Step#2:Changethesubnetmasktobinary.Step#3:UsethebooleanoperatorANDtocombinethetwo.Step#4:Convertthenetworkbinaryaddresstodotteddecimal.FiguringSubnetNetworkAddressesIPHost 20SubnetMask 10101100.00010000.00000010.0111100011111111.11111111.11111111.0000000010101100.00010000.00000010.00000000ThisisthesubnetnetworkaddressItcanhelpdeterminepath.ANDInordertofindthenetworkIDofasubnet,theroutermusttaketheIPaddress,andthesubnetmask,andlogically,ANDthemtogetherGivenandneeding8usablesubnets,findthesubnetworknumbers,therangesofhostnumbers,andsubnetworkbroadcastnumbers.Practice:IPAddressingProblemsIPAddressisaclassC.Defaultsubnetmaskis.Weneedtoextendthenetworknumberbyenoughbitstogive8usablesubnets.Stealing2bitsyields2usablesubnets,stealing3bitsyields6usablesubnets,sowemuststeal4bitstoget14usablesubnets,ofwhichweneeded8.Thismakesthesubnetmask40.SotheNetworknumberis195.137.92.NNNNHHHHwhereNsstandfornetworkextensionbits(subnets)andHsstandforhostnumbers.Nextwemustnumberthesubnets;thereare16combinationsof4bitbinarynumbersbuttheyretaintheirplacevaluewithinthelastoctet.SolutionLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPPathdeterminationPathdeterminationTherouterusestochoosethenexthopinthepathforthepackettotraveltoitsdestinationbasedonthelinkbandwidth,hop,delay…IPaddressesIPaddressesareimplementedinsoftware,andrefertothenetworkonwhichadeviceislocated.Routersconnectnetworks,eachofwhichmusthaveauniquenetworknumberinorderforroutingtobesuccessful.TheuniquenetworknumberisincorporatedintotheIPaddressthatisassignedtoeachdeviceattachedtothatnetworkRouterInterfaceRouterfunctionRouterfunction(cont.1)Stripsoffthedatalinkheader,carriedbytheframe.(ThedatalinkheadercontainstheMACaddressesofthesourceanddestination.)

Routerfunction(cont.2)Examinesthenetworklayeraddresstodeterminethedestinationnetwork.Routerfunction(cont.3)Consultsitsroutingtablestodeterminewhichofitsinterfacesitwillusetosendthedata,inorderforittoreachitsdestinationnetwork.Routerfunction(cont.4)SendthedataoutinterfaceB1,therouterwouldencapsulatethedataintheappropriatedatalinkframe.RouterInterfaceexampleInterfaceisarouter’sattachmenttoanetwork,itmayalsobereferredtoasaportinIProuting.Eachinterfacemusthaveaseparate,uniquenetworkaddress.IPaddressassignmentStatic

addressingConfigureeachindividualdevicewithanIPaddressYoushouldkeepverymeticulousrecords,becauseproblemscanoccurifyouuseduplicateIPaddresses.Dynamic

addressingThereareafewdifferentmethodscanbeusedtoassignIPaddressesdynamically:RARP:ReverseAddressResolutionProtocol.BOOTP:BOOTstrapProtocol.DHCP:DynamicHostConfigurationProtocol.Dynamicaddressing:

RARPMAC: KnownIP: UnknownRARPRequestRARPReplyRARPserverDynamicaddressing:

BOOTPMAC: KnownIP: UnknownUDPBroadcastUDPBroadcastBOOTPserverMAC1–IP1MAC2–IP2MAC3–IP3IPAddressGatewayIPofserverVendor-specificDynamicaddressing:

DHCPMAC: KnownIP: UnknownDHCPDiscoverUDPBroadcastDHCPOfferUDPUnicastDHCPserverIP1IP2IP3BroadcastDHCPRequestUnicastDHCPAckIPAddressGatewayIPofserversAndmore…Layer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPAddressResolutionProtocolInorderfordevicestocommunicate,thesendingdevicesneedboththeIPaddressesandtheMACaddressesofthedestinationdevices.ARPenablesacomputertofindtheMACaddressofthecomputerthatisassociatedwithanIPaddress.AddressResolutionProtocolARPTableRAMA.B.C.1.2.3A.B.C.7.8.9A.B.C.4.5.6ABCARPoperationARPTable:?MACA.B.C.1.2.3MAC?IPIPDataA.B.C.1.2.3A.B.C.7.8.9A.B.C.4.5.6ABCARPoperation:

ARPrequestMACA.B.C.1.2.3MACff.ff.ff.ff.ff.ffIPIPWhatisyourMACAddr?A.B.C.1.2.3A.B.C.7.8.9A.B.C.4.5.6ABCARPoperation:

CheckingMACA.B.C.1.2.3MACff.ff.ff.ff.ff.ffIPIPWhatisyourMACAddr?ThatismyIPA.B.C.1.2.3A.B.C.7.8.9A.B.C.4.5.6ABCARPoperation:

ARPreplyMACA.B.C.7.8.9MACA.B.C.1.2.3IPIPThisismyMACAddrA.B.C.1.2.3A.B.C.7.8.9A.B.C.4.5.6ABCARPoperation:

CachingARPTable:A.B.C.7.8.9–MACA.B.C.1.2.3MACA.B.C.7.8.9IPIPDataARP:

DestinationlocalARP:InternetworkCommunicationHowtocommunicatewithdevicesthatarenotonthesamephysicalnetworksegment?DefaultgatewayProxyARPDefaultgatewayInorderforadevicetocommunicatewithanotherdeviceonanothernetwork,youmustsupplyitwithadefaultgateway.AdefaultgatewayistheIPaddressoftheinterfaceontherouterthatconnectstothenetworksegmentonwhichthesourcehostislocated.Inorderforadevicetosenddatatotheaddressofadevicethatisonanothernetworksegment,thesourcedevicesendsthedatatoadefaultgateway.

ProxyARPProxyARPisavariationoftheARP.Inthecasethesourcehostdoesnothaveadefaultgatewayconfigured.ARPReplyARP:

DestinationnotlocalARPFlowchartSendDatatoadeviceSendDataSendanARPrequestGetanARPreply

IstheMACaddressinmyARPcacheNYLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPConnectionorientednetworkservicesAconnectionisestablishedbetweenthesenderandtherecipientbeforeanydataistransferred.CircuitswitchedConnection-orientedvs.circuitswitched.However,thetwotermsarenotthesameConnection-oriented:establishaconnectionwiththerecipient,first,andthenbeginthedatatransfer.Allpacketstravelsequentiallyacrossthesamechannel,ormorecommonly,acrossthesamevirtualcircuit.ConnectionlessnetworkservicesTheytreateachpacketseparately.IPisaconnectionlesssystem.PacketswitchedConnectionlessnetworkvs.packetswitched.Thetwotermsarenotthesame,eitherWhenthepacketspassfromsourcetodestination,theycan:Switchtodifferentpaths.Arriveoutoforder.Devicesmakethepathdetermination

foreachpacketbasedonavarietyofcriteria.Someofthecriteriamaydifferfrompackettopacket.Layer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPNetworkprotocoloperationRoutedprotocolProtocolsthatprovidesupportforthenetworklayerarecalledroutedorroutableprotocols.IPisanetworklayerprotocol,andbecauseofthat,itcanberoutedoveraninternetwork.Non-routableprotocolNon-routableprotocolsareprotocolsthatdonotsupportLayer3.Themostcommonofthesenon-routableprotocolsisNetBEUI.NetBEUIisasmall,fast,andefficientprotocolthatislimitedtorunningononesegment.AddressingofaroutableprotocolRoutingtableE0E2E1Classification#1:

StaticandDynamicStaticroutes:Thenetworkadministratormanuallyentertheroutinginformationintherouter.Dynamicroutes:Routerscanlearntheinformationfromeachotheronthefly.Usingroutingprotocoltoupdateroutinginformation.RIP,IGRP,EIGRP,OSPF…Staticvs.dynamicroutesStaticroutes:Forhidingpartsofaninternetwork.Totestaparticularlinkinanetwork.Formaintainingroutingtableswheneverthereisonlyonepathtoadestinationnetwork.Dynamicroutes:Maintenanceofroutingtable.Timelydistributionofinformationintheformofroutingupdates.Reliesonroutingprotocoltoshareknowledge.Routerscanadjusttochangingnetworkconditions.RoutingprotocolRoutingprotocolsdeterminethepathsthatroutedprotocolsfollowtotheirdestinations.Routedvs.RoutingprotocolRoutingprotocolsdeterminehowroutedprotocolsareroutedClassification#2:

IGPandEGP

Dynamicroutes.InteriorGatewayProtocols(RIP,IGRP,EIGRP,OSPF):Beusedwithinanautonomoussystem,anetworkofroutersunderoneadministration,likeacorporatenetwork,aschooldistrict'snetwork,oragovernmentagency'snetwork.ExteriorGatewayProtocols(EGP,BGP):Beusedtoroutepacketsbetweenautonomoussystems.

IGP

IGPvs.EGPEGPClassification#3:

DVPandLSPDistance-VectorProtocols(RIP,IGRP):Viewnetworktopologyfromneighbor’sperspective.Adddistancevectorsfromroutertorouter.Frequent,periodicupdates.Passcopyofroutingtablestoneighborrouters.LinkStateProtocols(OSPF):Getscommonviewofentirenetworktopology.Calculatestheshortestpathtootherrouters.Event-triggeredupdates.Passeslinkstateroutingupdatestootherrouters.DistancevectorroutingLinkstateroutingRIP(RouteInformationProtocol)Mostpopular.InteriorGatewayProtocol.DistanceVectorProtocol.Onlymetricisnumberofhops.Maximumnumberofhopsis15.Updatesevery30seconds.Doesn’talwaysselectfastestpath.Generateslotsofnetworktraffic.RIPv2isanimprovedversionofRIPv1IGRP(InteriorGatewayRouteProtocol)andEIGRP(EnhancedIGRP)Ciscoproprietary.InteriorGatewayProtocol.DistanceVectorProtocol.Metriciscomposeofbandwidth,load,delayandreliability.Maximumnumberofhopsis255.Updatesevery90seconds.EIGRPisanadvancedversionofIGRP,thatishybridroutingprotocol.OSPF(OpenShortestPathFirst)OpenShortestPathFirst.InteriorGatewayProtocol.LinkStateProtocol.Metriciscomposeofcost,speed,traffic,reliability,andsecurity.Event-triggeredupdates.Layer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPClassfulrouting&VLSMClassfulroutingClassfulroutingprotocolsrequirethatasinglenetworkusethesamesubnetmask.Example:networkmustusejustonesubnetmasksuchas.VLSM—Variable-LengthSubnetMasksVLSMissimplyafeaturethatallowsasingleautonomoussystemtohavenetworkswithdifferentsubnetmasks.VLSMWithVLSM,anetworkadministratorcanusealongmaskonnetworkswithfewhosts,andashortmaskonsubnetswithmanyhosts.IfaroutingprotocolallowsVLSM:usea30-bitsubnetmaskonnetworkconnections,52a24-bitmaskforusernetworks,Or,evena22-bitmask,,fornetworkswithupto1000users.WhyusetheVLSMVLSMallowsanorganizationtousemorethanonesubnetmaskwithinthesamenetworkaddressspace.ImplementingVLSMisoftenreferredtoas"subnettingasubnet",andcanbeusedtomaximizeaddressingefficiency.VLSMisoneofthemodificationsthathashelpedtobridgethegapbetweenIPv4andIPv6.VLSMAdvantages:

EfficientuseofIPaddressesBetterrouteaggregationSupportVLSMRoutingProtocol:OpenShortestPathFirst(OSPF)IntegratedIntermediateSystemtoIntermediateSystem(IntegratedIS-IS)EnhancedInteriorGatewayRoutingProtocol(EIGRP)RIPv2Staticrouting.AWasteofSpaceInthepast,ithasbeenrecommendedthatthefirstandlastsubnetnotbeused.ButwecanusedtheSubnet0fromCiscoIOSver12.0.FromIOSver12.0,theCmand:

router(config)#noipsubnet-zeroAWasteofSpace00

*****24(11100000)Host-idsubnet-idHowtousetheVLSMVLSM:AnExampleVLSM:AnExampleAclassCaddressof/24hasbeenallocated.Perth,Sydney,andSingaporehaveaWANconnectiontoKualaLumpur(KL).Perthrequires60hosts.KLrequires28hosts.SydneyandSingaporeeachrequire12hosts.TocalculateVLSMsubnetsandtherespectivehostsallocatethelargestrequirementsfirstfromtheaddressrange.Requirementslevelsshouldbelistedfromthelargesttothesmallest.VLSM:AnExampleStep1InthisexamplePerthrequires60hosts.Use6bitssince26

–2=62usablehostaddresses.Thus2bitswillbeusedfromthe4thOctettorepresenttheextended-network-prefixof/26andtheremaining6bitswillbeusedforhostaddresses.ApplyingVLSMonaddress/24gives:0hhhhhh/2692(11000000)/24（Subnetted）/26(00000000)Perth(60hosts)/26(NetworkAddress)/26/26……

1/262/263/26(BroadcastAddress)4/26(01000000)Unused（Subnetted）28/26(10000000)Unused92/26

(11000000)UnusedSeenextslideVLSM:AnExampleStep2KLrequires28hosts.Thenextavailableaddressafter3/26is4/26.Since28hostsarerequired,5bitswillbeneededforthehostaddresses,25

–2=30usablehostaddresses.Thus5bitswillberequiredtorepresentthehostsand3bitswillbeusedtorepresenttheextended-networkprefixof/27.ApplyingVLSMonaddress4/26gives:10hhhhh/2724(11100000)From4/264/27(01000000)KL(28hosts)4/27(NetworkAddress)5/276/27……

4/275/276/27(BroadcastAddress)6/27(01100000)Unused（Subnetted）28/27(10000000)Unused60/27(10100000)Unused92/27(11000000)Unused24/27(11100000)UnusedSeenextslideVLSM:AnExampleStep3NowSydneyandSingaporerequire12hostseach.Thenextavailableaddressstartsfrom6/27.Since12hostsarerequired,4bitswillbeneededforthehostaddresses,24=16,16–2=14usableaddresses.Thus4bitsarerequiredtorepresentthehostsand4bitsfortheextended-network-prefixof/28.ApplyingVLSMonaddress6/27gives:110hhhh/2840(11110000)From6/276/28Sydney(12hosts)6/28(NetworkAddress)7/288/28

……

09/2810/2811/28(BroadcastAddress)12/28Singapore(12hosts)12/28(NetworkAddress)13/2814/28……

26/2827/2828/28(BroadcastAddress)

28/28Unused（Subnetted）44/28Unused...…Unused40/28UnusedSeenextslideVLSM:AnExampleStep4NowallocateaddressesfortheWANlinks.RememberthateachWANlinkwillrequiretwoIPaddresses.Thenextavailablesubnetis28/28.Since2networkaddressesarerequiredforeachWANlink,2bitswillbeneededforhostaddresses,22

–2=2usableaddresses.Thus2bitsarerequiredtorepresentthelinksand6bitsfortheextended-network-prefixof/30.ApplyingVLSMon28/28gives:11000

hh/3052(11111100)From28/2828/30Perth–KL28/30(NetworkAddress)29/3030/3031/30(BroadcastAddress)32/30Sydney–KL32/30(NetworkAddress)33/3034/3035/30(BroadcastAddress)36/30Singapore–KL36/30(NetworkAddress)37/3038/3039/30(BroadcastAddress)40/30Unused44/30Unused…

…UnusedVLSM:AnExampleItisimportanttorememberthatonlyunusedsubnetscanbefurthersubnetted.Ifanyaddressfromasubnetisused,thatsubnetcannotbefurthersubnetted.VLSM:AnExampleRouteAggregationTheuseofClasslessInterDomainRouting(CIDR)andVLSMnotonly