Unit9TheInternetThemostnotableexampleofaninternetistheInternet(notetheuppercaseⅠ),whichoriginatedfromresearchprojectsgoingbacktotheearly1960s.Thegoalwastodeveloptheabilitytolinkavarietyofcomputernetworkssothattheycouldfunctionasaconnectedsystemthatwouldnotbedisruptedbylocaldisasters.MostofthisoriginalworkwassponsoredbytheU.S.governmentthroughtheDefenseAdvancedResearchProjectsAgency(DARPA-pronouncedDAR-pa).Overtheyears,thedevelopmentoftheInternetshiftedfromadefenseprojecttoanacademicresearchproject,andtodayitislargelyacommercialundertakingthatlinksaworldwidecombinationofWANs,MANs,andLANsinvolvingmillionsofcomputers.Ⅰ.Internetarchitecture一、因特网体系结构TheestablishmentofdomainsisoverseenbytheInternetCorporationforAssignedNamesandNumbers(ICANN),whichisanonprofitcorporationestablishedtocoordinatethenamingofdomainsandtheassignmentofInternetaddresses.ToestablishadomainontheInternet,thedomainmustfirstberegisteredviaoneofthecompanies,calledregistrars,thathavebeendelegatedforthispurposebyICANN.Onceadomainhasbeenregistered,itcanbeattachedtotheexistingInternetbymeansofarouterthatconnectsoneofthenetworksinthedomaintoanetworkalreadyintheInternet.Thisparticularrouterisoftenreferredtoasthedomain’sgateway,inthatitrepresentsthedomain’sgatetotherestoftheInternet.一个域一旦注册，即可以使用一个路由器连接到现有的因特网上。这个路由器将该域中的一个网络与已经在因特网内的一个网络相连接。这个特定的路由器常被称为该域的网关，因为它相当于该域通往因特网其余部分的大门。Fromthepointofviewofasingledomain,theportionoftheInternetlyingoutsideofitsgatewayissometimescalledthecloud,inreferencetothefactthatthestructureoftheInternetoutsidethedomain’sgatewayisoutofthedomain’scontrolandisofminimalconcerntotheoperationofthedomainitself.Anymessagebeingtransmittedtoadestinationwithinthedomainishandledwithinthedomain;anymessagebeingtransmittedtoandestinationoutsidethedomainisdirectedtowardthegatewaywhereitissentoutintothecloud.从单个域的角度看，位于其网关外面的那部分因特网有时被称为“云”，这是指以下的事实：其网关外面的因特网结构不受其控制，而且与其本身的操作也没有什么关系。传输给该域内一个目的地的任何报文都在该域内处理，传输给该域外一个目的地的任何报文都首先送至网关，然后由网关向外发送到云里。Ifone“stood”atadomain’sgatewayand“lookedout”intotheclou