外文翻译--互联网Internet.doc -- 5 元

1InternetTheInternetalsoknownastheNetistheworldslargestcomputernetwork.AndwhatisanetworkYoumayask.Acomputernetworkisbasicallyabunchofcomputersbookedtogethersomehow.Inconcept,itssortoflikearadioorTVnetworkthatconnectabunchofradioorTVstations.Somecomputernetworksconsistofacentralcomputerandabunchofremotestationsthatreporttoitacentralairlinereservationcomputer,forexample,withthousandsofterminalsatairportsandtravelagencies.Other,includingtheInternet,permitanycomputeronthenetworktocommunicatewithanyother.TheInternetisprobablythemostopennetworkintheworld.ThousandsofcomputersprovidefacilitiesthatareavailabletoanyonewhohasNetaccess.Thissituationisunusualmostnetworksareveryrestrictiveinwhattheyallowuserstodoandrequireapecificarrangementsandpasswordsforeachservice.Althoughafewpayservicesexist,thevastmajorityofInternetservicesarefreeforthetaking,onceyouconnected.IfyoudontalreadyhaveaccesstotheInternetthroughyourcompany,yourschool,youllprobablyhavetopayforaccesstotheInternetbyusingInternetaccessprovider.HowdoIgettotheInternetWelldependingonyourlifestyle,includingyourjoboryourschool,checkingouttheInternetcanbetriviallyeasyormoderatelydifficult.1ModemAmodemisalittledevicethatenablesdatafromonecomputertotraveltoanothercomputerbyusingordinarytelephonelines.Thiscutelittleboxgoesbetweenyourandyourphonelineandbetweenthephonelineandthecomputerontheotherend.modemarebuiltintosomecomputer,soifyouvenotsurewhetheroneisinyourcomputer,ask.Ifyouneedtobuyamodem,findthecheapestfaxmodemthatrunsat14400hps.Youcanpaymoreforthingsthatgofaster,butthatspeedisntreallypassedontoyoubyusingyourordinaryphonelinejustnow.Makesurethatyourmodemcomeswithsomecommunicationssoftwarethatsappropriateforthecomputerandversionofoperatingsystemyoureusing.TheInternetisouttherewithtonsofexcitingthingstodo,butyouhavetoitthroughyourschoolorwork.ThefolkswhoprovideaccesstotheInternetgenerallyforafeearecalledInternetServiceProviders.2OnlineOnlineserviceareservicesprovidedtoyoubyacomputerorcomputersinteractively.Theyincludebutarenotlimitedtocomputerizedbanking,shopping,dating,entertainment,andstudy.Theymayormaynothaveanythingtodowiththe2Internet,suchasCompuServe,existedindependentlyoftheInternet.TheyalloweduserstoexchangeelectronicmailamongotherCompuServeusersbutfailedtoreachouttotouchanyoneelse.Nowmostoftheseservicesdoprovideemailaccessbeyondtheirinsulatedclienthas.Thisisafarcry,however,fromrealInternetaccessandcapability.3BulletinboardsBBSElectronicbulletinboardsystemsabbreviatedasBBSprovideonlineservicesgenerallyonasmallerscale.Bulletinboardsystemisoftenlocal,oftenverycheap,andsometimesevenfreetocallers.Theymightprovideemail,cheatingonlineconversationswithotherusers,forumsaboutspecialinterests,games,ads,andlots,lotsmore.GatewayThefirstquestioniswhetheryourPChasanativeconnectiontotheInternetorwhetheritgoesthroughsomeothertypeofnetwork.Ifithasanativeconnection,yourPCrunsnetworksoftwarethathandlestheTCP/IPnetworkprotocolstheInternetuses.ThealternativeisthatyourPCisrunninganotherkindofnetworksoftwaremostoftenNovellsNetwareandisattachedtoagatewaysystemthatspeaksNetwareononesideandTCP/IPontheother.ItsalsopossibletoloadupbothTCP/IPandsomeotherkindofnetworksoftwareonthesamePCandrunthemonthesamephysicalnetworkcablesatthesametime.IfyoualreadyhavealargeNetwork,yoursystemmanagercanloadupasingleNetwaregatewaythatalltheotherPCsonthenetworkcansee.IfeachPCistorunTCP/IPitself,yournetworkmanagerhastoloadTCP/IPoneachPC,whichisagreatdealmorework.TCP/IPworksbetterinheterogeneousnetworksthatis,networksmadeupoflotsofdifferentkindsofcomputers.NovellNetwareisgreatonnetworksofPCsandlessgreatonotherkindsofcomputersbecauseNetwareatthispointworksonfarfewerkindsofcomputersthanTCP/IPdoes.EthernetTokenAssumethatyourPCrunsTCP/IPbyitselfandnotbywayofantherkindofnetwork.HowisitconnecttotherestoftheNetThethreemajorchoicesareEthernet,TokenRing,andseriallines.ThemostcommonwaytohookupPCstonetworksiswithEthernet,akindofnetworkthatisfastandcheapbutlimitedtoatotaldistanceoflessthanamile.forlargernetwork,apersoncanhooktogetherseveralEthernetnetworkbyusingdevicescalledbridgesandrouters.TokenRingAfewyearsback,IBMdecidedthatbecauseeveryothercomputermanufacturerintheworldwasusingEthernet,itwouldavoidtheheartacheofexcessivecompatibilitybycomingupwithitsownnetworkcalledTokenRing.FurioustechnicaldebatesragedonabouttherelativemeritsofEthernetandTokenRing,whichwecansummarizebysayingthatthedifferencesare95percentpolitical.Token3RingusesacleverrectangularconnectorthathastheadvantageovermanyversionofEthernetinthatitcomesunpluggedonlywhenyouwantitto.TheInternethasmorethanamillioncomputersattachedtoit.HowdoyoufindtheoneyouwantEachmachineonthenetisidentifiedbyanumberandaname.Eachmachineonthenetcallahosthasanumberassignedtoidentifyittootherhosts,sortoflikeaphonenumber.Thenumbersareinfourparts,suchas123.45.67.89.Mosthostsalsohavenames,whicharemucheasiertorememberthannumbersare.Thenameshavemultiplepartsseparatedbydotschico.iecc.com,forexample.Somehostshavemorethanonename,butitdoesntmatterwhichofthemyouuse.EachnetworkintheInternethasrulesaboutwhatkindsofnetworktrafficemail,terminalsessions,andotherconnectionsitallows.Youshouldknowtherulesthatapplytothenetworkornetworksyouusetoavoidgettingthenetworkmanagersmailatyou.Anycomputerofanykind,fromthesmallesttothelargest,attachedtotheInternetiscalledahost.SomehostsaregiantmainframesorsupercomputersthatprovideservicestothousandsofuserssomearelittleworkstationsorPCswithoneuserandsomearespecializedcomputers,likerouters,whichconnectonenetworktoanother,orterminalservers.ButfromtheInternetspointofview,theyreallhosts.Eachmachineisassignedahostnumber,whichissortoflikeaphonenumber.Beingcomputers,thekindofnumbershostslikeare32bitbinarynumbers.Forexample,10001100101110100101000100000001Thatsnotverymemorable.Tomakethenumberslightlyeasiertoremember,itsbrokenupintofour8bitgroups,andtheneachgroupistranslatedintoadecimalequivalent.Soournumberturnsinto140.186.81.1InternethostnumbersarealsodividedintotwopartsThefirstpartisthenetworknumber,andthesecondpart,thelocalpart,isahostnumberonthatparticularnetwork.Inthecaseofourcomputer,140.186.81.1meansnetworknumber140.186,andlocalhostnumberonthatnetwork81.1.Becausesomenetworkshavemanymorehostsonthemthanotherdo,networksaredividedintothreesizes.InlargenetworksClassA,thefirstofthefournumbersisthenetworknumber,andthelastthreearethelocalpart.InmediumnetworksClassB,thefirsttwonumbersarethenetworknumber,andthelasttwoarethelocalpart.InsmallnetworksClassC,thefirstthreenumbersarethenetworknumber,andthelastisthelocalpart.Thefirstofthefournumberstellsyouinwhichclassthenetworkis.Frequently,anorganizationthathasasinglenetworknumberwantstosetupitscomputersinternallyonmultiplenetworks.Allthecomputersinasingledepartment,forexample,areusuallyattachedtogetheronasinglenetwork,withsomesortofconnection4linkingtogetherdepartmentnetworks.ButadheringtothewaytheInternetwasoriginallysetupwouldmeanthatanoutfitwith25internalnetworkswouldhavetoget25differentnetworknumbersforthem.Itmeantthateverytimeacompanysetupanewinternalnetwork,ithadtoapplyforanewnetworknumber.TherestoftheInternetworldhadtoputthatnetworknumberintheirtablessothattheyknewhowtoroutemessagestoit.Clearly,somethinghadtobedone.Thatsomethingiscalledasubnet.Allthatmeansisthatonenetworkcanbedividedintopiecescalledsubnets.Onasubnet,partofwhatwouldnormallybethehostnumberbecomespartofthenetworknumber.Innetwork140.186,forexample,thethirdnumberinthehostnumberisthesubnetnumber,soformachine140.186.81.1,thesubnetnumberis140.186.81,andthehostnumberis1.Thisenablesplentyoflocalnetworkstobeinstalledweuseonly90ofthe254possiblesubnets,andasfarastheoutsideworldisconcerned,theresstillonlythesinglenetwork140.186toworryabout.Inpractice,allbutthesmallestnetworksaresubnetTed.Afeworganizationshaveanoppositeproblem.TheyhavetoomanycomputersforaClassCnetwork,morethan254,butnowherenearenoughtojustifyaClassB.Inthiscase,theorganizationcangetablockofadjacentnetworknumbersandtreatpartofthenetworknumberasahostnumber,aprocesscalledsupernetting.Normalpeopleusenames,notnumbers,soInternethostandusuallyreferredtobyname,notbynumber.Toavoidacrisisofnamingcreativity,thesolutionwastogotomultipartnamesaschemegrandlyknownastheDomainNamesystemorDNS.Hostnamesareastringofwordsseparatedbydots.Therightmostpartofanameiscalleditszone.ThethreeletterzonesaresetupbytypeoforganizationsuchasfollowingTableZoneMeaningcomCommercialorganizationseduEducationalinstitutionsgovGovernmentbodiedanddepartmentsintInternationalorganizationsmostlyNATOatthemomentmilMilitarysitesnetNetworkingorganizationsorgAnythingelsethatdoesntfitelsewhere,suchasprofessionalsocietiesTwoletterzonenamesareorganizedgeographically.EachzonecorrespondstoacountryorOtherPoliticalEntity.Forexample,thecountryforCanadaisCA,soasiteatYorkUniversityinCanadaiscallednexus,yorku,ca.Ifyouhaveoneortwomachines,itseasiertogetageographicname.Ifyouhavemorethanthat,itseasiertogetanorganizationalname,whichletsyouadministernameswithinyourorganizationyourself.WhattheInternetdoes,basically,istransmitdatafromonecomputertoanther.If