外文翻译--互联网Internet.doc

1InternetTheInternetalsoknownastheNetistheworldslargestcomputernetwork.“Andwhatisanetwork”?Youmayask.Acomputernetworkisbasicallyabunchofcomputersbookedtogethersomehow.Inconcept,itssortoflikearadioorTVnetworkthatconnectabunchofradioorTVstations.Somecomputernetworksconsistofacentralcomputerandabunchofremotestationsthatreporttoit-acentralairlinereservationcomputer,forexample,withthousandsofterminalsatairportsandtravelagencies.Other,includingtheInternet,permitanycomputeronthenetworktocommunicatewithanyother.TheInternetisprobablythemostopennetworkintheworld.ThousandsofcomputersprovidefacilitiesthatareavailabletoanyonewhohasNetaccess.Thissituationisunusual-mostnetworksareveryrestrictiveinwhattheyallowuserstodoandrequireapecificarrangementsandpasswordsforeachservice.Althoughafewpayservicesexist,thevastmajorityofInternetservicesarefreeforthetaking,onceyouconnected.IfyoudontalreadyhaveaccesstotheInternetthroughyourcompany,yourschool,youllprobablyhavetopayforaccesstotheInternetbyusingInternetaccessprovider.“HowdoIgettotheInternet?”Welldependingonyourlifestyle,includingyourjoboryourschool,checkingouttheInternetcanbetriviallyeasyormoderatelydifficult.(1)ModemAmodemisalittledevicethatenablesdatafromonecomputertotraveltoanothercomputerbyusingordinarytelephonelines.Thiscutelittleboxgoesbetweenyourandyourphonelineandbetweenthephonelineandthecomputerontheotherend.modemarebuilt-intosomecomputer,soifyouvenotsurewhetheroneisinyourcomputer,ask.Ifyouneedtobuyamodem,findthecheapestfaxmodemthatrunsat14400hps.Youcanpaymoreforthingsthatgofaster,butthatspeedisntreallypassedontoyoubyusingyourordinaryphonelinejustnow.Makesurethatyourmodemcomeswithsomecommunicationssoftwarethatsappropriateforthecomputerandversionofoperatingsystemyoureusing.TheInternetisouttherewithtonsofexcitingthingstodo,butyouhavetoitthroughyourschoolorwork.ThefolkswhoprovideaccesstotheInternet(generallyforafee)arecalledInternetServiceProviders.(2)On-lineOn-lineserviceareservicesprovidedtoyoubyacomputerorcomputersinteractively.Theyincludebutarenotlimitedtocomputerizedbanking,shopping,dating,entertainment,andstudy.Theymayormaynothaveanythingtodowiththe2Internet,suchasCompuServe,existedindependentlyoftheInternet.TheyalloweduserstoexchangeelectronicmailamongotherCompuServeusersbutfailedtoreachouttotouchanyoneelse.Nowmostoftheseservicesdoprovidee-mailaccessbeyondtheirinsulatedclienthas.Thisisafarcry,however,fromrealInternetaccessandcapability.(3)Bulletinboards(BBS)Electronicbulletinboardsystems(abbreviatedasBBS)provideon-lineservicesgenerallyonasmallerscale.Bulletinboardsystemisoftenlocal,oftenverycheap,andsometimesevenfreetocallers.Theymightprovidee-mail,cheating(on-lineconversationswithotherusers),forumsaboutspecialinterests,games,ads,andlots,lotsmore.GatewayThefirstquestioniswhetheryourPChasanativeconnectiontotheInternetorwhetheritgoesthroughsomeothertypeofnetwork.Ifithasanativeconnection,yourPCrunsnetworksoftwarethathandlestheTCP/IPnetworkprotocolstheInternetuses.ThealternativeisthatyourPCisrunninganotherkindofnetworksoftware(mostoftenNovellsNetware)andisattachedtoagatewaysystemthatspeaksNetwareononesideandTCP/IPontheother.ItsalsopossibletoloadupbothTCP/IPandsomeotherkindofnetworksoftwareonthesamePCandrunthemonthesamephysicalnetworkcablesatthesametime.IfyoualreadyhavealargeNetwork,yoursystemmanagercanloadupasingleNetwaregatewaythatalltheotherPCsonthenetworkcansee.IfeachPCistorunTCP/IPitself,yournetworkmanagerhastoloadTCP/IPoneachPC,whichisagreatdealmorework.TCP/IPworksbetterinheterogeneousnetworks-thatis,networksmadeupoflotsofdifferentkindsofcomputers.NovellNetwareisgreatonnetworksofPCsandlessgreatonotherkindsofcomputersbecauseNetwareatthispointworksonfarfewerkindsofcomputersthanTCP/IPdoes.EthernetTokenAssumethatyourPCrunsTCP/IPbyitselfandnotbywayofantherkindofnetwork.HowisitconnecttotherestoftheNet?ThethreemajorchoicesareEthernet,TokenRing,andseriallines.ThemostcommonwaytohookupPCstonetworksiswithEthernet,akindofnetworkthatisfastandcheapbutlimitedtoatotaldistanceoflessthanamile.(forlargernetwork,apersoncanhooktogetherseveralEthernetnetworkbyusingdevicescalledbridgesandrouters.TokenRingAfewyearsback,IBMdecidedthatbecauseeveryothercomputermanufacturerintheworldwasusingEthernet,itwouldavoidtheheartacheofexcessivecompatibilitybycomingupwithitsownnetworkcalledTokenRing.FurioustechnicaldebatesragedonabouttherelativemeritsofEthernetandTokenRing,whichwecansummarizebysayingthatthedifferencesare95percentpolitical.Token3RingusesacleverrectangularconnectorthathastheadvantageovermanyversionofEthernetinthatitcomesunpluggedonlywhenyouwantitto.TheInternethasmorethanamillioncomputersattachedtoit.Howdoyoufindtheoneyouwant?Eachmachineonthenetisidentifiedbyanumberandaname.Eachmachineonthenet(callahost)hasanumberassignedtoidentifyittootherhosts,sortoflikeaphonenumber.Thenumbersareinfourparts,suchas9.Mosthostsalsohavenames,whicharemucheasiertorememberthannumbersare.Thenameshavemultiplepartsseparatedbydots(,forexample).Somehostshavemorethanonename,butitdoesntmatterwhichofthemyouuse.EachnetworkintheInternethasrulesaboutwhatkindsofnetworktraffic(e-mail,terminalsessions,andotherconnections)itallows.Youshouldknowtherulesthatapplytothenetwork(ornetworks)youusetoavoidgettingthenetworkmanagersmailatyou.Anycomputerofanykind,fromthesmallesttothelargest,attachedtotheInternetiscalledahost.Somehostsaregiantmainframesorsupercomputersthatprovideservicestothousandsofusers；somearelittleworkstationsorPCswithoneuser；andsomearespecializedcomputers,likerouters,whichconnectonenetworktoanother,orterminalservers.ButfromtheInternetspointofview,theyreallhosts.Eachmachineisassignedahostnumber,whichissortoflikeaphonenumber.Beingcomputers,thekindofnumbershostslikeare32-bitbinarynumbers.Forexample,10001100101110100101000100000001Thatsnotverymemorable.Tomakethenumberslightlyeasiertoremember,itsbrokenupintofour8-bitgroups,andtheneachgroupistranslatedintoadecimalequivalent.SoournumberturnsintoInternethostnumbersarealsodividedintotwoparts:Thefirstpartisthenetworknumber,andthesecondpart,thelocalpart,isahostnumberonthatparticularnetwork.Inthecaseofourcomputer,meansnetworknumber140.186,andlocalhostnumber(onthatnetwork)81.1.Becausesomenetworkshavemanymorehostsonthemthanotherdo,networksaredividedintothreesizes.Inlargenetworks(ClassA),thefirstofthefournumbersisthenetworknumber,andthelastthreearethelocalpart.Inmediumnetworks(ClassB),thefirsttwonumbersarethenetworknumber,andthelasttwoarethelocalpart.Insmallnetworks(ClassC),thefirstthreenumbersarethenetworknumber,andthelastisthelocalpart.Thefirstofthefournumberstellsyouinwhichclassthenetworkis.Frequently,anorganizationthathasasinglenetworknumberwantstosetupitscomputersinternallyonmultiplenetworks.Allthecomputersinasingledepartment,forexample,areusuallyattachedtogetheronasinglenetwork,withsomesortofconnection4linkingtogetherdepartmentnetworks.ButadheringtothewaytheInternetwasoriginallysetupwouldmeanthatanoutfitwith25internalnetworkswouldhavetoget25differentnetworknumbersforthem.Itmeantthateverytimeacompanysetupanewinternalnetwork,ithadtoapplyforanewnetworknumber.TherestoftheInternetworldhadtoputthatnetworknumberintheirtablessothattheyknewhowtoroutemessagestoit.Clearly,somethinghadtobedone.Thatsomethingiscalledasubnet.Allthatmeansisthatonenetworkcanbedividedintopiecescalledsubnets.Onasubnet,partofwhatwouldnormallybethehostnumberbecomespartofthenetworknumber.Innetwork140.186,forexample,thethirdnumberinthehostnumberisthesubnetnumber,soformachine,thesubnetnumberis140.186.81,andthehostnumberis1.Thisenablesplentyoflocalnetworkstobeinstalled(weuseonly90ofthe254possiblesubnets),andasfarastheoutsideworldisconcerned,theresstillonlythesinglenetwork140.186toworryabout.Inpractice,allbutthesmallestnetworksaresubnetTed.Afeworganizationshaveanoppositeproblem.TheyhavetoomanycomputersforaClassCnetwork,morethan254,butnowherenearenoughtojustifyaClassB.Inthiscase,theorganizationcangetablockofadjacentnetworknumbersandtreatpartofthenetworknumberasahostnumber,aprocesscalledsupernetting.Normalpeopleusenames,notnumbers,soInternethostandusuallyreferredtobyname,notbynumber.Toavoidacrisisofnamingcreativity,thesolutio