第4章：OSI層次：網(wǎng)絡(luò)層

OSILayer3:

NetworkLayerRoutingandAddressingLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPLayer3ResponsibilitiesMovedatathroughnetworksUseahierarchicaladdressingscheme(opposedtoMACaddressing,whichisflat)SegmentnetworkandcontrolflowoftrafficReducecongestionTalktoothernetworksLayer3DevicesRoutersInterconnectnetworksegmentsornetworksMakelogicaldecisionsbasedonIPaddressesDeterminebestpathSwitchpacketsfromincomingportstooutgoingportsLayer3:NetworkLayerOverviewoftheNetworkLayerIPAddressesandSubnetsLayer3DevicesARPProtocolNetworkLayerServicesRoutedandRoutingProtocolsVLSMICMPMobileIPLayer3Packet/DatagramHeaderincludessourceANDdestinationaddresses固定部分可變部分04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特?cái)?shù)據(jù)部分首部傳送IP數(shù)據(jù)報(bào)首部可變部分首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特?cái)?shù)據(jù)部分首部傳送IP數(shù)據(jù)報(bào)固定部分首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特?cái)?shù)據(jù)部分首部傳送IP數(shù)據(jù)報(bào)固定部分可變部分首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分版本——占4bit，指IP協(xié)議的版本目前的IP協(xié)議版本號(hào)為4(即IPv4)首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分首部長(zhǎng)度——占4bit，可表示的最大數(shù)值是15個(gè)單位(一個(gè)單位為4字節(jié))因此IP的首部長(zhǎng)度的最大值是60字節(jié)。首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分服務(wù)類型——占8bit，用來獲得更好的服務(wù)這個(gè)字段以前一直沒有被人們使用首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分總長(zhǎng)度——占16bit，指首部和數(shù)據(jù)之和的長(zhǎng)度，單位為字節(jié)，因此數(shù)據(jù)報(bào)的最大長(zhǎng)度為65535字節(jié)。總長(zhǎng)度必須不超過最大傳送單元MTU。

首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分標(biāo)識(shí)(identification)占16bit，它是一個(gè)計(jì)數(shù)器，用來產(chǎn)生數(shù)據(jù)報(bào)的標(biāo)識(shí)。首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分標(biāo)志占3bit，最高位為0MF為0表示最后一個(gè)分片首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分片偏移(12bit)指出：較長(zhǎng)的分組在分片后某片在原分組中的相對(duì)位置。片偏移以8個(gè)字節(jié)為偏移單位。偏移=0/8=0偏移=0/8=0偏移=1400/8=175偏移=2800/8=350140028003799279913993799需分片的數(shù)據(jù)報(bào)數(shù)據(jù)報(bào)片1首部數(shù)據(jù)部分共3800字節(jié)首部1首部2首部3字節(jié)0數(shù)據(jù)報(bào)片2數(shù)據(jù)報(bào)片314002800字節(jié)0IP數(shù)據(jù)報(bào)分片的舉例首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分生存時(shí)間(8bit)記為TTL(TimeToLive)數(shù)據(jù)報(bào)在網(wǎng)絡(luò)中可通過的路由器數(shù)的最大值。首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分協(xié)議(8bit)字段指出此數(shù)據(jù)報(bào)攜帶的數(shù)據(jù)使用何種協(xié)議以便目的主機(jī)的IP層將數(shù)據(jù)部分上交給哪個(gè)處理過程運(yùn)輸層網(wǎng)絡(luò)層首部TCPUDPICMPIGMPOSPF數(shù)據(jù)部分IP數(shù)據(jù)報(bào)協(xié)議字段指出應(yīng)將數(shù)據(jù)部分交給哪一個(gè)進(jìn)程首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分首部檢驗(yàn)和(16bit)字段只檢驗(yàn)數(shù)據(jù)報(bào)的首部不包括數(shù)據(jù)部分。這里不采用

CRC

檢驗(yàn)碼而采用簡(jiǎn)單的計(jì)算方法。發(fā)送端接收端16bit字116bit字2置為全0檢驗(yàn)和16bit字n16bit反碼算術(shù)運(yùn)算求和……取反碼數(shù)據(jù)報(bào)首部IP數(shù)據(jù)報(bào)16bit檢驗(yàn)和16bit字116bit字216bit檢驗(yàn)和16bit字n16bit反碼算術(shù)運(yùn)算求和16bit結(jié)果……取反碼數(shù)據(jù)部分若結(jié)果為0,則保留；否則，丟棄該數(shù)據(jù)報(bào)數(shù)據(jù)部分不參與檢驗(yàn)和的計(jì)算首部04816192431版本標(biāo)志生存時(shí)間協(xié)議標(biāo)識(shí)服務(wù)類型總長(zhǎng)度片偏移填充首部檢驗(yàn)和源地址目的地址可選字段（長(zhǎng)度可變）比特首部長(zhǎng)度01234567DTRC未用優(yōu)先級(jí)數(shù)據(jù)部分比特固定部分可變部分源地址和目的地址都各占4字節(jié)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