计网-第四章作业

Chapter44.1,4.2,4.3节后作业1.(R3)Whatarethetwomostimportantnetwork-layerfunctionsinadatagramnetwork?Whatarethethreemostimportantnetwork-layerfunctionsinavirtual-circuitnetwork?答：forwardingandrouting.forwarding,routingandconnectionsetup.2.(R2)Whatisthedifferencebetweenroutingandforwarding?答：Forwardingisaboutmovingapacketfromarouter’sinputlinktotheappropriateoutputlink.Routingisaboutdeterminingtheend-to-routesbetweensourcesanddestinations.3.(R9)Describehowpacketlosscanoccuratinputports.Describehowpacketlossatinputportscanbeeliminated(withoutusinginfinitebuffers).答：Thequeuesizeattheinputportgrowslargebecauseofslowswitchingfabricspeedandexhaustingrouter’sbufferspace.Itcanbeeliminatediftheswitchingfabricspeedisatleastntimesasfastastheinputlinespeed,wherenisthenumberofinputports.4.4,4.5,4.6,4.7节后作业4.(R12)Whatisthe32-bitbinaryequivalentoftheIPaddress223.1.3.27?答：110111110000000100000011000110115.DoroutershaveIPaddresses?Ifso,howmany?答：Yes.Theyhaveoneaddressforeachinterface.6.ConsiderFigure4.34.StartingwiththeoriginaltableinD,supposethatDreceivesfromAthefollowingadvertisement.WillthetableinDchange?Ifsohow?答：No.IftheadvertisementsaidthatAwereonly4hopsawayfromzbywayofC,thenDwouldindeedchangeitsforwardingtable.7.(P8)Consideradatagramnetworkusing32-bithostaddresses.Supposearouterhasfivelinks,numbered0through4,andpacketsaretobeforwardedtothelinkinterfacesasfollows:DestinationAddressRangeLinkInterface11100000000000000000000000000000Through01110000000000000111111111111111111100000000000010000000000000000Through11110000000000001111111111111111111100000000000100000000000000000Through21110000011111111111111111111111111100001000000000000000000000000Through311100001111111111111111111111111Otherwise4a.Provideaforwardingtablethathasfiveentries,useslongestprefixmatching,andforwardspacketstothecorrectlinkinterfaces.b.Describehowyourforwardingtabledeterminestheappropriatelinkinterfacefordatagramswithdestinationaddresses:110010001001000101010001010101011110000010101101110000110011110011100001100000000001000101110111答：a.IPLinkinterface1110000000000000011100000000000011111000002111000013Otherwise4b.Prefixmatchforfirstaddressis4thentry:linkinterface3Prefixmatchforsecondaddressis2ndentry:linkinterface1Prefixmatchforfirstaddressis3rdentry:linkinterface28.(P9)Consideradatagramnetworkusing4bithostaddresses.Supposearouteruseslongestprefixmatchingandhasthefollowingforwardingtable:PrefixMatchInterface000011102113Foreachofthefourinterfaces,givetheassociatedrangeofdestinationhostaddressesandthenumberofaddressesintherange.答：DestinationAddressRangesNumberofaddressLinkInterface00000000~0011111164001000000~0111111164110000000~1011111164211000000~111111116439.(P11)Considerarouterthatinterconnectsthreesubnets:Subnet1,Subnet2,andSubnet3.Supposealloftheinterfacesineachofthesethreesubnetsarerequiredtohavetheprefix220.2.240/20.AlsosupposethatSubnet1isrequiredtosupportupto2000interfaces,andSubnets2and3areeachrequiredtosupportupto1000interfaces.Providethreenetworkaddresses(oftheforma.b.c.d/x)thatsatisfytheseconstraints.答：223.1.17.0/25223.1.17.128/26223.1.17.192/2610.(P14)Considerasubnetwithprefix101.101.101.64/26.GiveanexampleofoneIPaddress(offormxxx.xxx.xxx.xxx)thatcanbeassignedtothisnetwork.SupposeanISPownstheblockofaddressesoftheform101.101.128/17.Supposeitwantstocreatefoursubnetsfromthisblock,witheachblockhavingthesamenumberofIPaddresses.Whataretheprefixes(offorma.b.c.d/x)forthefoursubnets?答：TheIPaddressinrangeisfrom101.101.101.64to101.101.101.128Thefourequalsizesubnets(extends18and19)101.101.128/19101.101.160/19101.101.192/19101.101.224/1911.(P15)ConsiderthetopologyshowninFigure4.17.Denotethethreesubnetswithhosts(startingclockwiseatl2:00))asNetworksA,B,andC.DenotethesubnetswithouthostsasNetworksD,E,andF.a.Assignnetworkaddressestoeachofthesesixsubnets,withthefollowingconstraints:Alladdressesmustbeallocatedfrom214.97.254/23;SubnetAshouldhaveenoughaddressestosupport250interfaces;SubnetBshouldhaveenoughaddressestosupport120interfaces;andSubnetCshouldhaveenoughaddressestosupport120interfaces.Ofcourse,subnetsD,EandFshouldeachbeabletosupporttwointerfaces.Foreachsubnet,theassignmentshouldtaketheforma.b.c.d/xora.b.c.d/x-e.f.g.h/y.b.Usingyouranswertopart(a),providetheforwardingtables(usinglongestprefixmatching)foreachofthethreerouters.答：a.From214.97.254/23,theassignmentsareSubnetA:214.97.255/24(28=256addresses)SubnetB:214.97.254.0/25-214.97.254.0/29(27-8=120addresses)SubnetC:214.97.254.128/25(27=128addresses)SubnetD:214.97.254.0/31(21=2addresses)SubnetE:214.97.254.2/31(21=2addresses)ABCDEFSubnetF:214.97.254.4/30(22=4addresses)b.Assumethatnodatagramshaverouterinterfaceasultimatedestinations.Also,labelD,E,Ffortheupper-right,bottom,andupper-leftinteriorsubnets,respectively.Router1:OutgoingInterfaceLongestPrefixMatchA110111110000000100000001F110111110000000100001001D110111110000000100000111Router2:OutgoingInterfaceLongestPrefixMatchF110111110000000100001001E110111110000000100001000C110111110000000100000010Router3:OutgoingInterfaceLongestPrefixMatchB11011111000000010000