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Appendix D.  Answers to Review Questions

Appendix D.  Answers to Review Questions

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Chapter1

ThefivelayersoftheTCP/IPprotocolsuitearethefollowing:

Physicallayer

Data-linklayer

Internet(orIP)layer

Host-to-hostlayer

Applicationlayer

1



Thephysicallayercontainstheprotocolsofthephysicalmedium.

Thedatalinklayercontainstheprotocolsthatcontrolthephysicallayer:howthe

mediumisaccessedandshared,howdevicesonthemediumareidentified,and

howdataisframedbeforebeingtransmittedonthemedium.

Theinternetlayercontainstheprotocolsthatdefinethelogicalgroupingofdata

linksintoanetworkandthecommunicationacrossthatnetwork.

Thehost-to-hostlayercontainstheprotocolsthatdefineandcontrolthelogical,

end-to-endpathsacrossthenetwork.

TheapplicationlayercorrespondstotheOSIsession,presentation,andapplication

layers.



2 ThemostcommonIPversionnowinuseisversion4.



Routersperformfragmentationwhenapacketislongerthanthemaximumpacket

length(MaximumTransmissionUnit,orMTU)supportedbyadatalinkontowhich

thepacketmustbetransmitted.Thedatawithinthepacketwillbebrokeninto

3

fragments,andeachfragmentwillbeencapsulatedinitsownpacket.Thereceiver

usestheIdentifierandFragmentOffsetfieldsandtheMFbitoftheFlagsfieldto

reassemblethefragments.



TheTimetoLive(TTL)fieldprevents"lost"packetsfrombeingpassedendlessly

throughtheIPnetwork.Thefieldcontainsan8-bitintegerthatissetbythe

originatorofthepacket.Eachrouterthroughwhichthepacketpasseswill

4

decrementtheintegerbyone.IfarouterdecrementstheTTLtozero,itwill

discardthepacketandsendanICMP"timeexceeded"errormessagetothe

packet'ssourceaddress.



ThefirstoctetruledeterminestheclassofanIPaddressasfollows:

ClassA:Thefirstbitofthefirstoctetisalways0.

ClassB:Thefirsttwobitsofthefirstoctetarealways10.

5



ClassC:Thefirstthreebitsofthefirstoctetarealways110.

ClassD:Thefirstfourbitsofthefirstoctetarealways1110.

ClassE:Thefirstfourbitsofthefirstoctetarealways1111.



TheA,B,andCIPaddressesarerecognizedindotteddecimalandbinaryas

follows:



6



Class BinaryRangeofFirstOctet



DecimalRangeofFirstOctet



A



0000000101111110



1126



B



1000000010111111



128191



C



1100000011011111



192223



AnIPaddressmaskidentifiesthenetworkpartofanIPaddress.Eachoneinthe

32-bitmaskmarksthecorrespondingbitintheIPaddressasanetworkbit.Azero

7 inthemaskmarksthecorrespondingbitintheIPaddressasahostbit.ABoolean

ANDisperformedinall32bitsoftheaddressandthemask;intheresult,all

networkbitsofthemaskwillberepeated,andallhostbitswillbechangedtozero.



AsubnetisasubgroupingofaclassA,B,orCIPaddress.Withoutsubnetting,the

networkpartofamajorclassA,B,orCIPaddresscanonlyidentifyasingledata

8 link.SubnettingusessomeofthehostbitsofamajorIPaddressasnetworkbits,

allowingthesinglemajoraddresstobe"subdivided"intomultiplenetwork

addresses.



Aclassfulroutingprotocolhasnowaytodifferentiatebetweentheall-zerossubnet

9 andthemajorIPaddress,andbetweentheall-onessubnetandtheall-hosts,allsubnetsbroadcastaddressofthemajorIPaddress.



10



ARP,orAddressResolutionProtocol,isafunctionthatmapstheIPaddressesof



interfacesonadatalinktotheircorrespondingMACidentifiers.



ProxyARPisafunctionofanIProuter.IftherouterhearsanARPrequest,and

Thedestinationnetworkorsubnetisintherouter'sroutingtable,and

11



Thetableindicatesthatthedestinationisreachableviaadifferentrouter

interfacethantheoneonwhichtheARPrequestwasreceived,

TherouterwillrespondtotheARPrequestwithitsownMACaddress.



AredirectisanIProuterfunction.Ifadevicehassentapackettotherouterand

theroutermustforwardthepackettoanext-hoprouteronthesamedatalink,the

12

routerwillsendaredirecttotheoriginatingdevice.Theredirectwillinformthe

devicethatitcanreachthenext-hoprouterdirectly.



TCP,orTransmissionControlProtocol,providesaconnection-orientedserviceover

13 theconnectionlessinternetlayer.UDP,orUserDatagramService,providesa

connectionlessservice.



Correctsequencingisaccomplishedwithsequencenumbers.Reliabilityis

14 accomplishedbyusingchecksums,acknowledgments,timers,andretransmissions.

Flowcontrolisaccomplishedbywindowing.



AMACidentifierisafixed-lengthbinaryinteger.IfIPusedMACidentifiersasthe

15 hostpartoftheIPaddress,subnettingwouldnotbepossiblebecausetherewould

benoflexibilityinusingsomeofthehostbitsasnetworkbits.



16







TheonlypurposeoftheUDPheaderistoaddfieldsforthesourceanddestination

portnumbers.



Chapter2

1 IPv6addressesare128bitsinlength.



2



IPv6addressesarerepresentedaseight16-bithexadecimalsegmentsseparated

bycolons.



ThetworulesforcompactingIPv6addressesare

3



a. Theleadingzeroesinany16-bitsegmentdonothavetobewritten.

b. Anysingle,continuousstringofoneormore16-bitsegmentsconsistingofall

zeroescanberepresentedwithadoublecolon.



4



Usingmorethanonedoublecolonambiguatestheaddress;theexactlengthof

eachstringofzeroescannotbedetermined.



5



Bothaddressesareallzeros.::/0isthedefaultaddress,whereas::/128isthe

unspecifiedaddress.



6



ThepartofaunicastIPv6addressthatspecifiesthehostistheInterfaceID,andit

isusually64bitsinlength.



7 TheSubnetIDoftheunicastIPv6addressis16bitslong.



8 AnIPv6addressbeginningwithFF80::/10isalink-localaddress.



9 Thisisaglobalunicastaddress,identifiedbythefirstthreebitsof001.



10



Ananycastaddressisanaddressthatrepresentsaserviceratherthanadevice,

andcanthereforeappearonmorethanonedevice.



11



Amulticastaddressisanaddressthatrepresentsagroupofdevicesratherthana

singledevice.



12 TheIPv6headeris40bytesinlength.



TheFlowLabelfield,bylabelingindividualflows(packetswiththesamesource



13 anddestinationaddressandthesamesourceanddestinationports)intheheader,

isintendedtoallowhighlygranularloadbalancingwithouthavingtopaya

performancepenaltyfromhavingtolookintothepacketpayload.



TheIPv6NextHeaderfieldcorrespondstotheIPv4ProtocolNumberfield.Itis

14 nameddifferentlybecausethevalueofthefieldmightspecifyafollowingprotocol

headeroritmightspecifyanIPv6extensionheader.



TheHopLimitfieldcorrespondstotheIPv4TimetoLive(TTL)field.Thenameis

changedbecauseroutershaveneverdecrementedthefieldaccordingtotransit

15

time;rather,everytransitrouterdecrementsthefieldby1,markingahopinstead

ofatransittime.



TheIPv6NextHeaderfieldisliketheIPv4ProtocolNumberfieldinthatitisan8bitfieldthatcan,ifthenextheaderisanupper-layerprotocolheader,specifythe

16 protocolnumber.ButitisdifferentfromtheProtocolNumberfieldinthatitcan

alsospecify,ifthenextheaderisanIPv6extensionheader,thatheader'stype

number.



17



ExtensionheadersmaketheIPv6headermoreefficientbybeingspecializedto

specificfunctionsandonlybeingincludedwhenthespecificfunctionisused.



18 TheNextHeadervalueofICMPv6(correspondingtoaProtocolNumber)is58.



AsidefromtheuseoftheFragmentextensionheader,thesignificantdifferenceof

IPv6fragmentationfromIPv4fragmentationisthatIPv6routersdonotfragment

19

packets.Itisuptotheoriginatinghosttoeitherfragmentpacketsorensurethat

nopacketitoriginatesistoolarge.



ThefiveICMPv6messagesusedbyNDPareRouterSolicitation(RS),Router

20 Advertisement(RA),NeighborSolicitation(NS),NeighborAdvertisement(NA),and

Redirect.



21



TheMflag,whenset,tellshoststouseDHCPv6toconfigureitsaddress.TheO

flagtellshoststousedDHCPv6tofindotherlinkparameters.



22



TheReachableTimerfieldspecifiesthetime,inmilliseconds,thatanodeshould

assumeaneighborisreachableafterthenodehasconfirmedreachability.



23



TheRetransmitTimerfieldspecifiestheperiod,inmilliseconds,thatanodeshould

waitbetweensuccessivetransmissionsofanNS.



ARouterLifetimevalueof0intheRAindicatesthattheoriginatingroutershould



24 notbeaddedtoahost'sdefaultrouterlist.



TheSflag,whenset,indicatesthattheNAwassentinresponsetoanNS.Twowayreachabilityisconfirmed,andaneighboraddresschangedtoReachablestate

25 intheneighborcache,onlyiftheNAisinresponsetoasolicitation;sothe

receptionofanNAwiththeSbitcleared,indicatingthatitisunsolicited,doesnot

changethestateofaneighborcacheentry.



StatefuladdressautoconfigurationreliesonDHCPv6toallocateanaddresstothe

host.StatelessaddressautoconfigurationusesRAstodetermineaprefixoflarger

26

scopethanlink-local,plusMAC-to-EUI64conversion,todetermineahost's

address.



MAC-to-EUI64conversioninsertsavalueof0xFFFEinthemiddleofaMAC

27 address,thenflipstheU/Lbitto1,tocreatea64-bitInterfaceIDfroma48-bit

MACaddress.



28 Duplicateaddressdetectionmustneverbeperformedonananycastaddress.



29



TheprefixFF02:0:0:0:0:1:FF00::/104isusedforsolicitednodemulticast

addresses.Itisprependedtothelast24bitsofanaddressthatisbeingsolicited.



30



IPv6usestheNDPfunctionNeighborAddressResolutioninsteadofARP,anda

neighborcacheinsteadofanARPcache.



AprivacyaddressisoneinwhichtheInterfaceIDisrandomlygenerated,and

changedbothatsomeregularperiodandwheneverahostacquiresanewprefix.

31 Itisusedinconjunctionwithanautoconfiguredpublicaddresstoensure

anonymityofthehost.Thepublicaddressisusedforreachability,buttheprivate

addressisusedasthesourceaddressofanypacketsthehostoriginates.



32



AnIncompletestateindicatesthatNeighborAddressResolutionfortheentryisin

progress.



33



AProbestateindicatesthatanNShasbeensenttoverifytwo-wayreachabilityof

aStaleentry,butarespondingNAhasnotyetbeenreceived.



NeighborUnreachabilityDetectionverifiestwo-wayreachabilityofaneighbor

eitherby"hints"fromanupper-layerprotocolthathasreceivedan

34

acknowledgmentofasentmessage,orbyactivelyprobingtheneighborwithan

NS.







Chapter3

Ataminimum,eachentryoftheroutingtablemustincludeadestinationaddress

1 andtheaddressofanext-hoprouteroranindicationthatthedestinationaddress

isdirectlyconnected.



2



Variablysubnettedmeansthattherouterknowsofmorethanonesubnetmaskfor

subnetsofthesamemajorIPaddress.



3



DiscontiguoussubnetsaretwoormoresubnetsofamajorIPnetworkaddressthat

areseparatedbyadifferentmajorIPaddress.



4 showiprouteisusedtoexaminetheroutingtableofaCiscorouter.



Thecommandshowipv6routedisplaystheIPv6routetable.Prefixes,prefix

5 lengths,andthenext-hopaddressoroutgoinginterfacearedisplayed,asarethe

administrativedistanceandroutemetric.



6



Thefirstbracketednumberistheadministrativedistanceoftheroutingprotocolby

whichtheroutewaslearned.Thesecondnumberisthemetricoftheroute.



Whenastaticrouteisconfiguredtoreferenceanexitinterfaceinsteadofanext7 hopaddress,thedestinationaddresswillbeenteredintotheroutingtableas

directlyconnected.



AsummaryrouteisasinglerouteentrythatpointstomultiplesubnetsormajorIP

8 addresses.Inthecontextofstaticroutes,summaryroutescanreducethenumber

ofstaticroutesthatmustbeconfigured.



Anadministrativedistanceisaratingofpreferenceforaroutingprotocolora

staticroute.Everyroutingprotocolandeverystaticroutehasanadministrative

9 distanceassociatedwithit.Whenarouterlearnsofadestinationviamorethan

oneroutingprotocolorstaticroute,itwillusetheroutewiththelowest

administrativedistance.



Afloatingstaticrouteisanalternativeroutetoadestination.Theadministrative

10 distanceissethighenoughthatthefloatingstaticrouteisusedonlyifamorepreferredroutebecomesunavailable.



Equal-costloadsharingdistributestrafficequallyamongmultiplepathswithequal

11 metrics.Unequal-costloadsharingdistributespacketsamongmultiplepathswith

differentmetrics.Thetrafficwillbedistributedinverselyproportionaltothecostof

theroutes.



IfaninboundinterfaceisconfiguredforCEF,thepacketwillbeswitchedwithCEF,

andCEFloadbalancingruleswillapply:perdestinationorperpacketforIPv4,

dependingupontheconfiguration,andperdestinationforIPv6.IfCEFisnot

12 configuredontheinboundinterface,theoutboundinterfacedeterminesthe

switchingmode,andthusthemethodofloadsharing.Ifanoutboundinterfaceis

fastswitched,perdestinationloadsharingisperformed.Ifaninterfaceisprocess

switched,perpacketloadsharingisperformed.



Arecursiveroutingtablelookupoccurswhenaroutercannotacquireallthe

informationitneedstoforwardapacketwithasingleroutingtablelookup.For

13 example,theroutermayperformonelookuptofindtheroutetoadestinationand

thenperformanotherlookuptofindaroutetothenext-hoprouterofthefirst

route.



Chapter4

1



Aroutingprotocolisa"language"thatroutersspeaktoeachothertoshare

informationaboutnetworkdestinations.



Ataminimum,aroutingprotocolshoulddefineproceduresfor

Passingreachabilityinformationaboutnetworkstootherrouters.

Receivingreachabilityinformationfromotherrouters.

2

Determiningoptimalroutesbasedonthereachabilityinformationithasand

forrecordingthisinformationinaroutetable.

Reactingto,compensatingfor,andadvertisingtopologychangesina

network.



3



Aroutemetric,alsocalledaroutecostoraroutedistance,isusedtodetermine

thebestpathtoadestination.Bestisdefinedbythetypeofmetricused.



4



Convergencetimeisthetimeagroupofrouterstakestocompletetheexchangeof

routinginformation.



Loadbalancingistheprocessofsendingpacketsovermultiplepathstothesame

destination.Fourtypesofloadbalancingare

Equalcost,perpacket.

5



Equalcost,perdestination.

Unequalcost,perpacket.

Unequalcost,perdestination.



Adistancevectorprotocolisaroutingprotocolinwhicheachroutercalculates

6 routesbasedontheroutesofitsneighborsandthenpassesitsroutestoother

neighbors.



Severalproblemsassociatedwithdistancevectorprotocolsare



Asusceptibilitytoincorrectroutinginformationbecauseofitsdependenceon

neighborsforcorrectinformation.

7



Slowconvergence.

Routeloops.

Countingtoinfinity.



8 Neighborsareroutersconnectedtothesamedatalink.



9



Routeinvalidationtimersdeleteroutesfromaroutetableiftheyexceedacertain

age.



Simplesplithorizondoesnotsendrouteinformationbacktothesourceofthe

10 routeinformation.Splithorizonwithpoisonedreversesendstheinformationback

tothesourcebutsetsthemetrictounreachable.



Countingtoinfinityoccurswhenroutesupdatearouteoveraloop;eachrouter

increasesthemetricoftherouteuntilthemetricreachesinfinity.Theeffectsof

11

countingtoinfinityarecontrolledbydefininginfinityasafairlylowmetricsothat

infinityisreachedfairlyquicklyandtherouteisdeclaredunreachable.



Holddowntimershelppreventroutingloops.Ifarouteisdeclaredunreachableorif

themetricincreasesbeyondacertainthreshold,arouterwillnotacceptanyother

12 informationaboutthatrouteuntiltheholddowntimerexpires.Thisapproach

preventstherouterfromacceptingpossiblybadroutinginformationwhilethe

networkisreconverging.



Adistancevectorroutersendsitsentireroutetable,butitonlysendsthetableto

directlyconnectedneighbors.Alinkstateroutersendsonlyinformationaboutits

directlyconnectedlinks,butitfloodstheinformationthroughoutthenetworking

13

area.DistancevectorprotocolsusuallyuseavariantoftheBellman-Fordalgorithm

tocalculateroutes,andlinkstateprotocolsusuallyuseavariantoftheDijkstra

algorithmtocalculateroutes.



14



Atopologicaldatabaseholdsthelinkstateinformationoriginatedbyallroutersin

thelinkstateroutingdomain.



Eachrouterfloodsalinkstateinformationadvertisementdescribingitslinks,the

statesofitslinks,andanyneighboringroutersconnectedtothoselinks,

throughoutthenetworkingarea.Allroutersstoreallreceivedcopiesofthelink

15

stateadvertisementinalinkstatedatabase.Eachroutercalculatesashortestpath

treefromtheinformationinthetopologicaldatabaseandentersroutesinits



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Appendix D.  Answers to Review Questions

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