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LessonUniformResourceLocators231.Canyoulistsometop-leveldomainnames?2.Whatdoes“www”standfor?Warm-upExercisesURL统一资源定位器hu

man-readable人们可读的machine-readable机器可读的IP（IP协议）top-level顶级unique独特，唯一left-most最左convert转换WordPreparationWhenyouusetheWeborsendane

-mailmessage,youuseadomainnametodoit.Forexample,theUniformResourceLocator(URL)"http://www.Yiren.com"(readash-t-t-p-colon-sla

sh-slash(ordoubleslash)-w-w-w-dot-….)containsthedomainnameYiren.com.Sodoesthise-mailaddress:example@Yiren.com.Everytimeyouuseadomainna

me,youusetheInternet'sDNSserverstotranslatethehuman-readabledomainnameintothemachine-readableIPaddress.Text每次使用域名时，要用Internet的DNS服务器将可读域名变成

机器可读的IP地址Top-leveldomainnames,alsocalledfirst-leveldomainnames,include.COM,.ORG,.NET,.EDUand.GOV.Withinevery

top-leveldomainthereisahugelistofsecond-leveldomains.Forexample,inthe.COMfirst-leveldomainthereis:YirenYahooMicrosoftEverynameinthe.COMtop-leveldom

ainmustbeunique.Theleft-mostword,likewww,isthehostname.Itspecifiesthenameofaspecificmachine(withaspecificIPaddress)inadomain.Ag

ivendomaincan,potentially,containmillionsofhostnamesaslongastheyarealluniquewithinthatdomain.Text给定域中可能有几百万个主机名，只要它们在域中

全部唯一DNSserversacceptrequestsfromprogramsandothernameserverstoconvertdomainnamesintoIPaddresses.Whenarequestcomesin,theDNSservercandooneoffourt

hingswithit:1.ItcananswertherequestwithanIPaddressbecauseitalreadyknowstheIPaddressfortherequesteddomain.2.ItcancontactanotherDNSserveran

dtrytofindtheIPaddressforthenamerequested.Itmayhavetodothismultipletimes.Text3.Itcansay,"Idon'tknowtheIPaddressforthedomainyourequest

ed,buthere'stheIPaddressforaDNSserverthatknowsmorethanIdo."4.Itcanreturnanerrormessagebecausetherequesteddomainnameisinvalidordoesn

otexist.TextURLInternetprotocol,datatransmissionstandard:thestandardthatcontrolstheroutingandstructureofdatatransmittedovertheInternetInternetad

dress:anaddressidentifyingthelocationofafileontheInternet,consistingoftheprotocol,thecomputeronwhichthe

fileislocated,andthefile’slocationonthatcomputer.FullformUniformResourceLocatorKeyTermsIPQuestionsCanyoulistsometop-leveldomainnames?Whatdoe

s“www”standfor?.Whenyouusetheweb,whatdoestheDNSserversdoforyou?WhydoesDNSserverreturnanerrormessage?ExerciseTranslatethefollowingsentencesin

toChineseWhenyouusetheWeborsendane-mailmessage,youuseadomainnametodoit.Withineverytop-leveldomainthereisahugelistofsecond-lev

eldomains.Everynameinthe.COMtop-leveldomainmustbeunique.Theleft-mostword,likewww,isthehostname.Itspecifiesthenameofasp

ecificmachine(withaspecificIPaddress)inadomain.ExerciseSimplewritingPleasedescribewhattheDNSserversdowhenusetheweb.E

xercise——THANKYOU！——