CCNA学习指南英文第七版第三章.docx

CCNA学习指南英文第七版第三章.docx

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CCNA学习指南英文第七版第三章

Chapter3

IntroductiontoTCP/IP

TheCCNAexamtopicscoveredinthischapterincludethefollowing:

∙Describehowanetworkworks

oDescribethepurposeandbasicoperationoftheprotocolsintheOSIandTCPmodels.

oIdentifyandcorrectcommonnetworkproblemsatlayers1,2,3,and7usingalayeredmodelapproach.

∙ImplementanIPaddressingschemeandIPServicestomeetnetworkrequirementsinamedium-sizeEnterprisebranchofficenetwork.

oDescribetheoperationandbenefitsofusingprivateandpublicIPaddressing.

The TransmissionControlProtocol/InternetProtocol（TCP/IP） suitewascreatedbytheDepartmentofDefense（DoD）toensureandpreservedataintegrityaswellasmaintaincommunicationsintheeventofcatastrophicwar.Soitfollowsthatifdesignedandimplementedcorrectly,aTCP/IPnetworkcanbeatrulydependableandresilientone.Inthischapter,I’llcovertheprotocolsofTCP/IP,andthroughoutthisbook,you’lllearnhowtocreateamarvelousTCP/IPnetwork—usingCiscorouters,ofcourse.

We’llbeginbytakingalookattheDoD’sversionofTCP/IPandthencomparethisversionanditsprotocolswiththeOSIreferencemodeldiscussedinChapter1,“Internetworking.”

OnceyouunderstandtheprotocolsusedatthevariouslevelsoftheDoDmodel,I’llcoverIPaddressingandthedifferentclassesofIPaddressesusedinnetworkstoday.

SubnettingwillbecoveredinChapter4,“EasySubnetting.”

Last,becauseIPv4addresstypesaresoimportanttounderstandingIPaddressing,aswellassubnettingandVariableLengthSubnetMasks（VLSMs）,anunderstandingofthevariousflavorsofIPv4addressesiscritical.I’llfinishthechapterwithvarioustypesofIPv4addressesthatyoujustmustknow.

InternetProtocolversion6willnotbediscussedinthischapter;thischapterwillfocussolelyonIPv4.IPv6willbecoveredinChapter15,“InternetProtocolVersion6（IPv6）.”Also,whenIdiscussInternetProtocolVersion4,you’llseeitwrittenasjustIP,nottypicallyIPv4.

IntroducingTCP/IP

BecauseTCP/IPissocentraltoworkingwiththeInternetandintranets,it’sessentialforyoutounderstanditindetail.I’llbeginbygivingyousomebackgroundonTCP/IPandhowitcameaboutandthenmoveontodescribingtheimportanttechnicalgoalsdefinedbytheoriginaldesigners.Afterthat,you’llfindouthowTCP/IPcomparestoatheoreticalmodel—theOpenSystemsInterconnection（OSI）model.

ABriefHistoryofTCP/IP

TCP/IPfirstcameonthescenein1973.Later,in1978,itwasdividedintotwodistinctprotocols:

TCPandIP.Then,in1983,TCP/IPreplacedtheNetworkControlProtocol（NCP）andwasauthorizedastheofficialmeansofdatatransportforanythingconnectingtoARPAnet,theInternet’sancestorthatwascreatedbyARPA,theDoD’sAdvancedResearchProjectsAgency,waybackin1957inreactiontotheSoviet’slaunchingofSputnik.ARPAwassoonredubbedDARPA,anditwasdividedintoARPAnetandMILNET（alsoin1983）;bothwerefinallydissolvedin1990.

Butcontrarytowhatyoumightthink,mostofthedevelopmentworkonTCP/IPhappenedatUCBerkeleyinNorthernCalifornia,whereagroupofscientistsweresimultaneouslyworkingontheBerkeleyversionofUNIX,whichsoonbecameknownastheBSD,orBerkeleySoftwareDistribution,seriesofUNIXversions.Ofcourse,becauseTCP/IPworkedsowell,itwaspackagedintosubsequentreleasesofBSDUNIXandofferedtootheruniversitiesandinstitutionsiftheyboughtthedistributiontape.Sobasically,BSDUnixbundledwithTCP/IPbeganassharewareintheworldofacademiaand,asaresult,becamethebasisofthehugesuccessandexponentialgrowthoftoday’sInternetaswellassmaller,privateandcorporateintranets.

Asusual,whatmayhavestartedasasmallgroupofTCP/IPaficionadosevolved,andasitdid,theU.S.governmentcreatedaprogramtotestanynewpublishedstandardsandmakesuretheypassedcertaincriteria.ThiswastoprotectTCP/IP’sintegrityandtoensurethatnodeveloperchangedanythingtoodramaticallyoraddedanyproprietaryfeatures.It’sthisveryquality—thisopen-systemsapproachtotheTCP/IPfamilyofprotocols—thatprettymuchsealeditspopularitybecauseitguaranteesasolidconnectionbetweenmyriadhardwareandsoftwareplatformswithnostringsattached.

TCP/IPandtheDoDModel

TheDoDmodelisbasicallyacondensedversionoftheOSImodel—it’scomposedoffour,insteadofseven,layers:

∙Process/Applicationlayer

∙Host-to-Hostlayer

∙Internetlayer

∙NetworkAccesslayer

Figure3-1 showsacomparisonoftheDoDmodelandtheOSIreferencemodel.Asyoucansee,thetwoaresimilarinconcept,buteachhasadifferentnumberoflayerswithdifferentnames.

WhenthedifferentprotocolsintheIPstackarediscussed,thelayersoftheOSIandDoDmodelsareinterchangeable.Inotherwords,theInternetlayerandtheNetworklayerdescribethesamething,asdotheHost-to-HostlayerandtheTransportlayer.

Figure3-1:

 TheDoDandOSImodels

AvastarrayofprotocolscombineattheDoDmodel’s Process/Applicationlayer tointegratethevariousactivitiesanddutiesspanningthefocusoftheOSI’scorrespondingtopthreelayers（Application,Presentation,andSession）.We’llbelookingcloselyatthoseprotocolsinthenextpartofthischapter.TheProcess/Applicationlayerdefinesprotocolsfornode-to-nodeapplicationcommunicationandalsocontrolsuser-interfacespecifications.

The Host-to-Hostlayer parallelsthefunctionsoftheOSI’sTransportlayer,definingprotocolsforsettinguptheleveloftransmissionserviceforapplications.Ittacklesissuessuchascreatingreliableend-to-endcommunicationandensuringtheerror-freedeliveryofdata.Ithandlespacketsequencingandmaintainsdataintegrity.

The Internetlayer correspondstotheOSI’sNetworklayer,designatingtheprotocolsrelatingtothelogicaltransmissionofpacketsovertheentirenetwork.IttakescareoftheaddressingofhostsbygivingthemanIP（InternetProtocol）address,andithandlestheroutingofpacketsamongmultiplenetworks.

AtthebottomoftheDoDmodel,the NetworkAccesslayer implementsthedataexchangebetweenthehostandthenetwork.TheequivalentoftheDataLinkandPhysicallayersoftheOSImodel,theNetworkAccesslayeroverseeshardwareaddressinganddefinesprotocolsforthephysicaltransmissionofdata.

TheDoDandOSImodelsarealikeindesignandconceptandhavesimilarfunctionsinsimilarlayers. Figure3-2 showstheTCP/IPprotocolsuiteandhowitsprotocolsrelatetotheDoDmodellayers.

Inthefollowingsections,wewilllookatthedifferentprotocolsinmoredetail,startingwiththeProcess/Applicationlayerprotocols.

TheProcess/ApplicationLayerProtocols

Inthefollowingsections,I’lldescribethedifferentapplicationsandservicestypicallyusedinIPnetworks.Thefollowingprotocolsandapplicationsarecovered:

∙Telnet

∙FTP

∙TFTP

∙NFS

∙SMTP

∙POP

∙IMAP4

∙TLS

∙SIP（VoIP）

∙RTP（VoIP）

∙LPD

∙XWindow

∙SNMP

∙SSH

∙HTTP

∙HTTPS

∙NTP

∙NNTP

∙SCP

∙LDAP

∙IGMP

∙LPR

∙DNS

∙DHCP/BootP

Figure3-2:

 TheTCP/IPprotocolsuite

Telnet

Telnet isthechameleonofprotocols—itsspecialtyisterminalemulation.Itallowsauseronaremoteclientmachine,calledtheTelnetclient,toaccesstheresourcesofanothermachine,theTelnetserver.TelnetachievesthisbypullingafastoneontheTelnetserverandmakingtheclientmachineappearasthoughitwereaterminaldirectlyattachedtothelocalnetwork.Thisprojectionisactuallyasoftwareimage—avirtualterminalthatcaninteractwiththechosenremotehost.

Theseemulatedterminalsareofthetext-modetypeandcanexecutedefinedproceduressuchasdisplayingmenusthatgiveuserstheopportunitytochooseoptionsandaccesstheapplicationsonthedupedserver.UsersbeginaTelnetsessionbyrunningtheTelnetclientsoftwareandthenloggingintotheTelnetserver.

FileTransferProtocol（FTP）

FileTransferProtocol（FTP） istheprotocolthatactuallyletsustransferfiles,anditcanaccomplishthisbetweenanytwomachinesusingit.ButFTPisn’tjustaprotocol;it’salsoaprogram.Operatingasaprotocol,FTPisusedbyapplications.Asaprogram,it’semployedbyuserstoperformfiletasksbyhand.FTPalsoallowsforaccesstobothdirectoriesandfilesandcanaccomplishcertaintypesofdirectoryoperations,suchasrelocatingintodifferentones.

AccessingahostthroughFTPisonlythefirststep,though.Usersmustthenbesubjectedtoanauthenticationloginthat’sprobablysecuredwithpasswordsandusernamesimplementedbysystemadministratorstorestrictaccess.Youcangetaroundthissomewhatbyadoptingtheusername anonymous—thoughwhatyou’llgainaccesstowillbelimited.

Evenwhenemployedbyusersmanuallyasaprogram,FTP’sfunctionsarelimitedtolistingandmanipulatingdirectories,typingfilecontents,andcopyingfilesbetweenhosts.Itcan’texecuteremotefilesasprograms.

TrivialFileTransferProtocol（TFTP）

TrivialFileTransferProtocol（TFTP） isthestripped-down,stockversionofFTP,butit’stheprotocolofchoiceifyouknowexactlywhatyouwantandwheretofindit,plusit’ssoeasytouseandit’sfasttoo!

Itdoesn’tgiveyoutheabundanceoffunctionsthatFTPdoes,though.TFTPhasnodirectory-browsingabilities;itcandonothingbutsendandreceivefiles.Thiscompactlittleprotocolalsoskimpsinthedatadepartment,sendingmuchsmallerblocksofdatathanFTP,andthere’snoauthenticationaswithFTP,soit’sevenmoreinsecure.Fewsitessupportitbecauseoftheinherentsecurityrisks.

NetworkFileSystem（NFS）

NetworkFileSystem（NFS） isajewelofaprotocolspecializinginfilesharing.Itallowstwodi