计算机网络专业译文关于IPv6.docx
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计算机网络专业译文关于IPv6.docx
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计算机网络专业译文关于IPv6
专业译文
原文:
《CiscoCertifiedNetworkAssociateStudyGuide.SixthEdition》的第740-747页
(作者:
ToddLammle出版社:
WileyPublishing,Inc.)
英语原文:
Ihopeyou'rereadytolearnaboutthenutsandboltsofInternetProtocolversion6(IPv6),becauseyou'regoingtogettherubonitinthischapter!
YoushouldhaveasolidholdonIPv4bynow,butifyouthinkyoucouldusearefresher,justpagebacktoChapter3,"Subnetting,VariableLengthSubnetMasks(VLSMs),andTroubleshootingTCP/IP."Andifyou'renotcrystalclearontheaddressproblemsinherenttoIPv4,youreallyshouldreviewChapter11,"NetworkAddressTranslation(NAT)”
PeoplerefertoIPv6as"thenext-generationInternetprotocol,"anditwasoriginallycreatedastheanswertoIPv4'sinevitable,loomingaddress-exhaustioncrisis.Thoughyou'veprobablyheardathingortwoaboutIPv6already,ithasbeenimprovedevenfurtherinthequesttobringustheflexibility,efficiency,capability,andoptimizedfunctionalitythatcantrulymeetoureverincreasingneeds.Thecapacityofitspredecessor,IPv4,palesincomparison-andthat'sthereasonitwilleventuallyfadeintohistorycompletely.
TheIPv6headerandaddressstructurehasbeencompletelyoverhauled,andmanyofthefeaturesthatwerebasicallyjustafterthoughtsandaddendumsinIPv4arenowincludedasfull-blownstandardsinIPv6.It'sseriouslywellequipped,poised,andreadytomanagethemind-blowingdemandsoftheInternettocome.
Ipromise-really-tomakethischapterprettypainless.Infact,youmightevenfindyourselfactuallyenjoyingit-Idefinitelydid!
BecauseIPv6issocomplexyetelegant,innovativeandchock-fulloffeatures,itfascinatesmelikesomeweirdcombinationofabrand-newLamborghiniandarivetingfuturisticnovel.Hopefullyyou'llexperiencethischapterasthecoolridethatIdidwritingit!
WhyDoWeNeedIPv6?
Well,theshortansweris,becauseweneedtocommunicate,andourcurrentsystemisn'treallycuttingitanymore-kindoflikehowthePonyExpresscan'tcompetewithairmail.Justlookathowmuchtimeandeffortwe'veinvestedincomingupwithslicknewwaystoconservebandwidthandIPaddresses.We'veevencomeupwithVariableLengthSubnetMasks(VLSMs)inourstruggletoovercometheworseningaddressdroughtIt'sreality-thenumberofpeopleanddevicesthatconnecttonetworksincreaseseachandeveryday.That'snotabadthingatall-we'refindingnewandexcitingwaystocommunicatetomorepeopleallthetime,andthat'sagoodthing.Infact,it'sabasichumanneed.Buttheforecastisn'texactlyblueskiesandsunshinebecause,asIalludedtointhischapter'sintroduction,IPv4,uponwhichourabilitytocommunicateispresentlydependent,isgoingtorunoutofaddressesforustouse.IPv4hasonlyabout4.3billionaddressesavailable-intheory,andweknowthatwedon'tevengettouseallofthose.Therereallyareonlyabout250millionaddressesthatcanbeassignedtodevices.Sure,theuseofClasslessInter-DomainRouting(CIDR)andNetworkAddressTranslation(NAT)hashelpedtoextendtheinevitabledearthofaddresses,butwewillrunoutofthem,andit'sgoingtohappenwithinafewyears.Chinaisbarelyonline,andweknowthere'sahugepopulationofpeopleandcorporationstherethatsurelywanttobe.Therearealotofreportsthatgiveusallkindsofnumbers,butallyoureallyneedtothinkabouttoconvinceyourselfthatI'mnotjustbeinganalarmististhefactthatthereareabout6.5billionpeopleintheworldtoday,andit'sestimatedthatjustover10percentofthatpopulationisconnectedtotheInternet-wow!
ThatstatisticisbasicallyscreamingatustheuglytruththatbasedonIPv4'scapacity,everypersoncan'tevenhaveacomputer-letalonealltheotherdevicesweusewiththem.Ihavemorethanonecomputer,andit'sprettylikelyyoudotoo.AndI'mnotevenincludinginthemixphones,laptops,gameconsoles,faxmachines,routers,switches,andamotherlodeofotherdevicesweuseeveryday!
SoIthinkI'vemadeitprettyclearthatwe'vegottodosomethingbeforewerunoutofaddressesandlosetheabilitytoconnectwitheachotherasweknowit.Andthat"something"justhappenstobeimplementingIPv6.
TheBenefitsandUsesofIPv6
Sowhat'ssofabulousaboutIPv6?
Isitreallytheanswertoourcomingdilemma?
IsitreallyworthittoupgradefromIPv4?
Allgoodquestions-youmayeventhinkofafewmore.Ofcourse,there'sgoingtobethatgroupofpeoplewiththetime-testedandwell-known"resistancetochangesyndrome,"butdon'tlistentothem.Ifwehaddonethatyearsago,we'dstillbewaitingweeks,evenmonthsforourmailtoarriveviahorseback.Instead,justknowthattheanswerisaresoundingYES!
NotonlydoesIPv6giveuslotsofaddresses(3.4x1038=definitelyenough),buttherearemanyotherfeaturesbuiltintothisversionthatmakeitwellworththecost,time,andeffortrequiredtomigratetoit.LaterinthechapterI'lltalkaboutallthateffortinthesectioncalled"MigratingtoIPv6."Init,I'llcoversomeofthetransitiontypesrequiredtomovefromversion4toversion6,andIpromiseyou'lldiscoverthatthehugebenefitsofmigratingwillvastlyoutweighanyassociatedcons.
Today'snetworks,aswellastheInternet,haveatonofunforeseenrequirementsthatsimplywerenotconsiderationswhenIPv4wascreated.We'vetriedtocompensatewithacollectionofadd-onsthatcanactuallymakeimplementingthemmoredifficultthantheywouldbeiftheywererequiredbyastandard.Bydefault,IPv6hasimproveduponandincludedmanyofthosefeaturesasstandardandmandatory.OneofthesesweetnewstandardsisIPSec-afeaturethatprovidesend-to-endsecuritythatI'llcoveriliChapter14,"WideAreaNetworks."Anotherlittlebeautyisknownasmobility,andasitsnamesuggests,itallowsadevicetoroamfromonenetworktoanotherwithoutdroppingconnections.
Butit'stheefficiencyfeaturesthatarereallygoingtorockthehouse!
Forstarters,theheaderinanIPv6packethavehalfthefields,andtheyarealignedto64bits,whichgivesussomeseriouslysouped-upprocessingspeed-comparedtoIPv4,lookupshappenatlightspeed!
MostoftheinformationthatusedtobeboundintotheIPv4headerwastakenout,andnowyoucanchoosetoputit,orpartsofit,backintotheheaderintheformofoptionalextensionheadersthatfollowthebasicheaderfields.
Andofcoursethere'sthatwholenewuniverseofaddresses(3.4x1038)wetalkedaboutalready.Butwheredidwegetthem?
DidthatCrissAngel-Mindfreakdudejustshowupand,Blammo?
Imean,thathugeproliferationofaddresseshadtocomefromsomewhere!
WellitjustsohappensthatIPv6givesusasubstantiallylargeraddressspace,meaningtheaddressisawholelotbigger-fourtimesbiggerasamatteroffact!
AnIPv6addressisactually128bitsinlength,andnoworries-I'mgoingtobreakdowntheaddresspiecebypieceandshowyouexactlywhatitlookslikecomingupinthesection"IPv6AddressingandExpressions."Fornow,letmejustsaythatallthatadditionalroompermitsmorelevelsofhierarchyinsidetheaddressspaceandamoreflexibleaddressarchitecture.Italsomakesroutingmuchmoreefficientandscalablebecausetheaddressescanbeaggregatedalotmoreeffectively.AndIPv6alsoallowsmultipleaddressesforhostsandnetworks.Thisisespeciallyimportantforenterprisesjonesingforavailability.Plus,thenewversionofIPnowincludesanexpandeduseofmulticastcommunication(onedevicesendingtomanyhostsortoaselectgroup),whichwillalsojoinintoboostefficiencyonnetworksbecausecommunicationswillbemorespecific.
IPv4usesbroadcastsveryprolifically,causingabunchofproblems,theworstofwhichisofcoursethedreadedbroadcaststorm-anuncontrolleddelugeofforwardedbroadcasttrafficthatcanbringanentirenetworktoitskneesanddevoureverylastbitofbandwidth.Anothernastythingaboutbroadcasttrafficisthatitinterruptseachandeverydeviceonthenetwork.Whenabroadcastissentout,everymachinehastostopwhatit'sdoingandrespondtothetrafficwhetherthebroadcastismeantforitornot.
Butsmileeveryone:
ThereisnosuchthingasabroadcastinIPv6becauseitusesmulticasttrafficinstead.Andtherearetwoothertypesofcommunicationaswell:
unicast,whichisthesameasitisinIPv4,andanewtypecalledanycast.Anycastcommunicationallowsthesameaddresstobeplacedonmorethanonedevicesothatwhentrafficissenttoonedeviceaddressedinthisway,itisroutedtothenearesthostthatsharesthesameaddress.Thisisjustthebeginning-we'llgetmoreintothevarioustypesofcommunicationinthesectioncalled"AddressTypes”
IPv6AddressingandExpressions
JustasunderstandinghowIPaddressesarestructuredandusediscriticalwithIPv4addressing,it'salsovitalwhenitcomestoIPv6.You'vealreadyreadaboutthefactthatat128bits,anIPv6addressismuchlargerthananIPv4address.Becauseofthis,aswellasthenewwaystheaddressesmanage.Butcanbeused,you'veprobablyguessedthatIPv6willbemorecomplicatedtonoworries!
AsIsaid,I'llbreakdownthebasicsandshowyouwhattheaddresslookslike,howyoucanwriteit,andwhatmanyofitscommonusesare.It'sgoingtobealittleweirdatfirst,butbeforeyouknowit,you'llhaveitnailed!
Solet'stakealookatFigure13.1,whichhasasampleIPv6addressbrokendownintosections.
FIGURE13.1IPv6addressexample
Soasyoucannowsee,theaddressistrulymuchlarger-butwhatelseisdifferent?
Well,first,noticethatithaseightgroupsofnumbersinsteadoffourandalsothatthosegroupsareseparatedbycolonsinsteadofperiods.A
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