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1、AnIntroductiontotheInternetNetworkingEnvironmentandSIMNET/DISbyJohnLocke(Internet:jxxl@cs.nps.navy.mil)ComputerScienceDepartment,NavalPostgraduateSchool1.THEPHYSICALDESIGNOFTHEINTERNETTheInternetistheworld’slargestdatanetwork.ItwasoriginallydevelopedbyBolt
2、BeranekandNewman,Inc.(BBN)inthelateseventiesundertheauspicesoftheDefenseAdvancedResearchProjectsAgency(DARPA).Thebasicoperationofthenetworkispacket-switching--thesendinghostdividesablockofdataintopackets,transmitseachpacketindividuallyandthereceivinghostre
3、assemblesthem.Inbetween,thepacketssharethesamenetworkbandwidthwithpacketsfromotherhosts.Notehowthisdiffersfromacircuit-switchednetworklikethephonesystem.Duringaphonecall,callerandreceivershareexclusiveuseofarangeofthenetworkbandwidth.Sothereisnooverlapbetw
4、eenthephonesystemandtheInternet,thecablingisseparate,theoperationisdifferent.However,thetwosystemsdointerfacewhereInternet-connectedsystemshavemodemsandprovideservices,likeelectronicmailtransfer,tonon-Internethosts.TheInternetwassonamedbecauseitdrewtogethe
5、ranumberofseparatenetworksintoasinglecohesivesystem.Amongothers,theseincludedtheMILNET,theNSFNET,andtheoriginaltestbed,theARPANET.BecauseoftheInternet’sdistributeddesign,newnetworkscanbeaddedtothesystemwithminimaleffort.NPS,forexample,ispartoftheBayAreaRes
6、earchNetwork(BARRNET),whichprimarilyservicestheSanFranciscoBayArea,butextendsasfarsouthasMontereyandtoDavis,Californiatothenortheast.Withinthesenetworksarelocallyadminis-teredentitieslikeNPS.Inturn,wearesubdividedintolocal-areanetworks(LANs)connectedtoacam
7、pusbackbonenetwork.Individualnetworks,largeandsmall,areconnectedbygateways,of-tenspecial-purposehardwarecalledrouters,whosepurposeistotransferpacketsfromonenet-worktoanother,ifitgetsthemclosertotheirdestination.Thefollowingfigureillustratestherelationshipbe
8、tweennetworksandgatewaysintheComputerScienceDepartment.NotethatbecauseoftheuniformdesignoftheInternet,anypartofitisami-crocosmofthewhole.ThehorizontallinesrepresentEthernetcables,acommon,butnotrequired,LANmed