Basicintroduction
TheDomainNameSystem(DNS)isasystemfornamingonlinemachinesontheInternet.Justlikevisitingafriend,youmustfirstknowhowotherpeople'shomesgo.WhenahostwantstoaccessanotherhostontheInternet,itmustfirstknowitsaddress.TheIPaddressinTCP/IPisanumberseparatedbyfour"."Composition(heretakingIPv4addressasanexample,IPv6addressisthesame),rememberingisalwaysnotasconvenientasname,sothedomainnamesystemisusedtomanagethecorrespondencebetweennameandIP.
AlthoughthenodesontheInternetcanbeuniquelyidentifiedbyIPaddressesandcanbeaccessedthroughIPaddresses,evenifthe32-bitbinaryIPaddressiswrittenasfourtensof0to255,It'sstilltoolongandhardtoremember.Therefore,peopleinventedthedomainname(DomainName),whichcanassociateanIPaddresswithasetofmeaningfulcharacters.Whenauservisitsawebsite,hecaneitherenterthewebsite’sIPaddressoritsdomainname.Foraccess,thetwoareequivalent.Forexample:theIPaddressofMicrosoft'sWebserveris207.46.230.229,anditscorrespondingdomainnameiswww.microsoft.com.Regardlessofwhethertheuserenters207.46.230.229orwww.microsoft.cominthebrowser,theycanaccessitsWebwebsite.
Acompany’sWebsitecanberegardedasitsportalontheInternet,andthedomainnameisequivalenttoitshouseaddress.Usuallythedomainnameusesthecompany’snameorabbreviation.Forexample,thedomainnameofMicrosoftmentionedaboveissimilarto:IBM'sdomainnameiswww.ibm.com,Oracle'sdomainnameiswww.oracle.com,Cisco'sdomainnameiswww.cisco.com,etc.Whenpeoplewanttovisitacompany'sWebsitewithoutknowingitsexactdomainname,theywillalwaysenterthecompanynameasatestfirst.However,adomainnameconsistingofacompany'snameorabbreviationmayalsoberegisteredbyothercompaniesorindividuals.Thereareevensomecompaniesorindividualsmaliciouslyregisteringalargenumberofdomainnamesconsistingofthenamesofwell-knowncompanies,andthenresellingthemtothesecompaniesathighpricesforprofit.Therehavebeensomearbitrationmeasuresfordomainnameregistrationdisputes,buttocontrolsuchphenomenafromthesource,acompletesetofrestrictionmechanismsisneeded.Thisisnotyetthere.Therefore,registeringadomainnameconsistingofone'sownnameassoonaspossibleshouldbesomethingthatanycompanyororganization,especiallythosefamouscompanies,mustpayattentionto.Somecompanieshavealreadydoneaprotectiveregistrationofdomainnamescomposedoftheirownfamousbrands.
Thenamespaceofthedomainnamesystem
Thehierarchicalstructureofthenamespace
Thenamespacereferstothecollectionofallpossiblenamesdefined.Thenamespaceofthedomainnamesystemishierarchical,similartoWindowsfilenames.Itcanberegardedasatreestructure.Thedomainnamesystemdoesnotdistinguishbetweennodesandleafnodesinthetree,butcollectivelyreferredtoasnodes.Differentnodescanusethesamemark.Themarkofallnodescanonlybecomposedof3typesofcharacters:26Englishletters(a~z),10Arabicnumerals(0-9)andEnglishhyphen(-),andthelengthofthemarkshallnotexceed22characters.Thedomainnameofanodeiscomposedofthemarkedconnectionsofallnodesfromthenodetotheroot,separatedbydots.Thedomainnameofthetop-levelnodeiscalledthetop-leveldomainname(TLD,Top-LevelDomain),thedomainnameofthesecond-levelnodeiscalledthesecond-leveldomainname,andsoon.
Distributionandmanagementofdomainnames
DomainnamesaremanagedbytheInternetCorporationforAssignedNamesandNumbers(ICANN,InternetCorporationforAssignedNamesandNumbers),whichisresponsiblefordomainnamesystemmanagementandIPaddressmanagement.Anon-profitorganizationestablishedforfunctionssuchasallocation,protocolparameterconfiguration,andmainserversystemmanagement.ICANNhassetupcorrespondingtop-leveldomainnamesfordifferentcountriesorregions,andthesedomainnamesusuallyconsistoftwoEnglishletters.Forexample:.ukfortheUnitedKingdom,.frforFrance,and.jpforJapan.Thetop-leveldomainnameinChinais.cn,andthedomainnamesunder.cnaremanagedbyCNNIC.
CNNICstipulatesthatthesecond-leveldomainnamecannotbeappliedforunderthe.cndomain,andthelengthofthethird-leveldomainnamemustnotexceed20characters,andthefollowingrestrictionsareimposedonthename:
①Theregistrationcontains"Domainnameswiththewords"CHINA","CHINESE","CN"and"NATIONAL"mustbeformallyapprovedbyrelevantnationaldepartments(referringtounitsabovetheministeriallevel).
②Thenamesofothercountriesorregions,foreigngeographicalnames,andinternationalorganizationnamesknowntothepublicshallnotbeused.
③Theuseofthefullnameorabbreviationoftheadministrativedivisionnameabovethecountylevel(includingthecountylevel)mustbeformallyapprovedbytherelevantpeople'sgovernmentatorabovethecountylevel(includingthecountylevel).
④Theindustrynameorthegeneralnameoftheproductshallnotbeused.
⑤EnterprisenamesorbrandnamesthathavebeenregisteredinChinabyothersmustnotbeused.
⑥Namesthatareharmfultothecountry,society,orpublicinterestshallnotbeused.
⑦Formallyapprovedbyrelevantstatedepartments(referringtounitsatorabovetheministeriallevel)andformallyapprovedbytherelevantpeople’sgovernmentsatorabovethecountylevel(includingcountylevel),itmeansthattherelevantagenciesmustissuewrittendocumentstoexpresstheirconsent.××××Unitregistration×××domainname.Forexample,ifyouwanttoapplyforthedomainnamebeijing.com.cn,youmustprovidetheapprovaloftheBeijingMunicipalPeople'sGovernment.
Top-leveldomainnames
Inadditiontorepresentingtop-leveldomainnamesofvariouscountries,ICANNinitiallydefined7top-leveldomainnames,whichare.com,.top,.edu,.gov,.mil,.net,.org;.com,.topareusedforenterprises,.eduisusedforeducationalinstitutions,.govisusedforgovernmentagencies,.milisusedformilitarydepartments,and.netisusedforInternetandinformationcentersEtc.,.orgisusedfornon-profitorganizations.
WiththedevelopmentoftheInternet,ICANNhasaddedtwocategoriesandatotalof7top-leveldomainnames,namely.aero,.biz,coop,.info,.museum,.name,.pro.Amongthem,.aero,.coop,and.museumarethreetop-leveldomainsforspecificindustriesorgroups:.aerorepresentstheairtransportindustry,.cooprepresentsthecollaborativeorganization,and.museumrepresentsthemuseum;.biz,.info,.name,.Prois4generictop-leveldomains:.bizmeansbusiness,.namemeanspersonal,.promeansaccountants,lawyers,physicians,etc.,and.infohasnospecificdirection.
Thecompositionofthedomainnamesystem
Resourcefile
IntheearlydaysoftheInternet,therewereonlyhundredsofhosts.Atthattime,thecorrespondencebetweendomainnamesandIPaddresseswassimplyrecordedinInahosts.txtfile,thisfileismaintainedbytheNetworkInformationCenter(NIC).AnyadministratorwhowantstoaddahostontheInternetshouldE-mailitsnameandaddresstotheNIC,andthiscorrespondencewillbemanuallyaddedtothehosts.txtfile.EachhostadministratorgoestotheNICtodownloadthelatesthosts.txtfileandputitontheirhost,andcompletestheupdateofthedomainnamelist.Domainnameresolutionisjustalocalprocessofcheckinglocalfiles.
WiththeexpansionofthenumberofhostsontheInternet,theoriginalmethodhasbeenunabletomeettherequirements.Theexistingdomainnamesystemwasputintouseinthe1980s.Thedomainnamesystemusesahierarchicalnamespace,andtheoriginalhugecorrespondencetableisdecomposedintodisjointsub-tablesdistributedintheInternet.Thesesub-tablesarecalledresourcefiles.
Resolution
Thehierarchicalstructureofthedomainnamesystemnamespacehasbeenexplainedabove,let’stakealookathowthisstructureisthesameasthedomainnameserver(DNS,DomainNameServer)ofthedomainnamesystem.Combinedtoachievedomainnameresolution.
First,accordingtothehierarchicalstructureofthedomainnamesystemdomainnamespace,itisdividedintodifferentareasaccordingtosubtrees.Eachareacanberegardedasamanageablepowerentityresponsibleforthispartofthenodeinthehierarchy.Forexample,thetop-levelareaoftheentiredomainismanagedbyICANN,andsomenationaldomainnamesandtheirsubordinatenodesconstitutetheirrespectiveareas.Forexample,the.cndomainismanagedbyCNNIC.The.cndomainisdividedintosmallerareas.Forexample,.fudan.edu.cnismanagedbytheFudanUniversityNetworkCenter.
Secondly,eachareamusthaveacorrespondingdomainnameserver,andtheinformationcontainedineachareaisstoredonthedomainnameserver.Therecanbetwoormoredomainnameserversinanarea,sothatevenifonedomainnameserverfails,theotherdomainnameservercanstillprovideinformationnormally.Adomainnameservercanalsomanagemultipleregionsatthesametime.Thedomainnameserverqueriesitsownresourcerecordcollectionafterreceivingtherequestfromtheuser,andreturnsthefinalanswerthattheuserwantstoget,orwhentherequiredanswerisnotfoundinitsownresourcerecordcollection,itreturnsapointertoanotherdomainnameserverPointer,theuserwillcontinuetomakerequeststothatdomainnameserver.Therefore,thedomainnameserverdoesnotneedtorecordtheinformationofallsubdomainsandhosts.Forthesubdomain(ifitexists),itonlyneedstoknowthedomainnameserverofthesubdomain.
Resourcerecordisadomainnametovaluebinding,whichincludesthefollowingfields:domainname,value,type,classification,andlifetime.Thedomainnamefieldandthevaluefieldareusedtoindicatethecontentoftheanalysisandtheresultoftheanalysis,respectively.Thetypefieldrepresentsthetypeofvalue:typeArepresentsthevaluefieldisanIPaddress,thatis,thefinalanswertheuserwants;typeNSrepresentsthevaluefieldisthedomainnameofanotherdomainnameserver,thedomainnameservercanknowhowtoresolvethedomainnamefieldThespecifieddomainname;thetypeisCNAME,thevaluefieldisanaliasofthehostspecifiedbythedomainname;thetypeisMX,thevaluefieldisthedomainnameofamailserverthatreceivesmailfromthedomainspecifiedbythedomainnamefield;thetypeisPTRUsedfordomainnamereversesolution,etc.Theclassificationfieldallowsotherrecordtypestobespecified.Thelifetimefieldisusedtoindicatethevalidityperiodoftheresourcerecord.Inordertoreducethedomainnameresolutiontime,thedomainnameserverwillcachesomeresourcerecordsfromotherdomainnameserversthathavebeenqueried.Becausetheseresourcerecordswillbecomeinvalidduetochanges,thedomainnameserverhassetalifetime,andexpiredresourcerecordswillbeclearedoutofthecache.
Therootdomainnameserverknowsthedomainnameserversofalltop-leveldomainnames,andcorrespondstoeachtop-leveldomainname.Ithastworesourcerecords:oneisanNSresourcerecord,thedomainnamefieldisthetop-leveldomainname,andthevaluefieldistheThedomainnameofthedomainnameserverresolvedbythetop-leveldomainname;theotherisAresourcerecord,whichisusedtoindicatetheIPaddresscorrespondingtothedomainnameofthedomainnameserver.Usingthesetworecordstogether,youcanknowwhichIPaddressthedomainnameservershouldcontinuetosearchforwhenresolvingacertaindomainnameunderthedomain.Thesecond-leveldomainnameserversimilarlystoresthepointersofeachthird-leveldomainnameserver.Thethird-leveldomainnameserverwillappearA,CNAME,MXandothertypesofresourcerecords.Eachdomainnameserverhasarecordoftheaddressoftherootdomainnameserver.
Finally,auserwhoneedsdomainnameresolutionfirstsendstheresolutionrequesttothelocaldomainnameserver.Ifthelocaldomainnameservercanresolve,theresultwillbeobtaineddirectly,otherwisethelocaldomainnameserverwillsendarequesttotherootdomainnameserver.Accordingtothepointerreturnedbytherootdomainnameserver,thedomainnameserverofthenextlayerisqueried,andsoon,andfinallytheIPaddressofthedomainnametoberesolvedisobtained.
Domainnamereversesolution
DomainnamereversesolutionreferstogivinganIPaddressandfindingoutitscorrespondingdomainname,whichisalsorealizedbyusingDNS.Forexample,supposeanIPaddresstobereversedis202.120.225.9,andthesystemrewritesitto9.225.120.202.in-addr.arpa,andthenqueriesitbymeansofdomainnameresolution.Thisrequiresaresourcerecordcorrespondingto9.225.120.202.in-addr.arpaonthelocaldomainnameserverofthequeriedhost,thetypeisPTR,andthevalueisitsdomainname.
ChinesewebsiteandChinesedomainname
Chinesewebsite
BecausetheInternetoriginatedintheUnitedStates,domainnamesarealsocomposedofEnglishletters.ForthisEnglishrepresentation,TheChinesearenotusedtoit,anditisdifficulttointegrate.Infact,thenamesofChinesecompaniesororganizationsregisteredonlineareoftenverydifferentfromtheirrealnames.Forexample,everyoneknowsthe"LiberationDaily",buttherearenotmanywhoknowitsdomainnamewww.jfdaily.com.ThetraditionaleconomyismergingwiththeInternet.Ifcustomerscannotdirectlyguesstheirdomainnamesbasedoncompanynamesandbrands,thenthecompany’soriginalbrandadvantagescannotbedirectlyextendedtotheInternet.ThisismostChinesecompanies,evenforeigncompaniesenteringthedomesticmarket.Anembarrassingproblemthatwillalwaysbeencountered.Evenmoreembarrassingisthat,inordernottobemaliciouslyregisteredorimpersonated,andtherebyaffectitsownimage,acompanymayhavetoregistermanysimilardomainnamesatthesametime,suchaswww.jiefangdaily.com,www.liberationdaily.com,etc.Thiswillbeasumofmoney.Alotofoverhead.
Inviewofthis,ChinahasbeguntoexploretheChinesecultureofnetworkaddresses.In1999,www.3721.comfirstputforwardtheconceptofChinesewebsiteinChina.UsingChineseURLs,usersnolongerneedtomemorizecumbersomeandlengthyEnglishdomainnameswhenvisiting,andtheydon’tneedtoenterwww.,.comandothersuffixes.ChinesecompanynamesorproductnamescanbedirectlyusedasURLs,whichgreatlyexpandsthebrand’sInfluence.Forexample,youcandirectlyuse"People'sDaily","XinhuaNewsAgency","TombRaider"andsoon.www.3721.comhasfurtherintroducedreal-nameonline,whichprovidesfouraccessmethods:ChineseURL,EnglishURL,PinyinURLanddigitalURL.
YouneedtoinstallaclientsoftwaretousetheChinesewebsite.Afterrunningthesoftware,aslongasyouentertheChinese,Pinyinprefix,oreventhestockcodeintheaddressbarofthebrowser,youcandirectlyreachthecorrespondingwebpagewithoutgoingtoanywebsitetosearch.
ChineseDomainNames
Inearly2000,CNNIClaunchedaChinesedomainnameregistrationtestsystem.TheMinistryofInformationIndustryissuedthe"NoticeontheManagementofChineseDomainNamesontheInternet"inNovember2000,whichregulatestheregistrationsystemofChinesedomainnames.InFebruary2001,CNNICannouncedonitswebsitethatthetop-levelChinesedomainnameserverhadbeguntoprovideresolutionservices.
CNNIC’sChinesedomainnamewillprovidetwosolutionsatthesametime:oneisaChinesedomainnameendingin.cn,andtheotherisapureChinesedomainname,whichisshapedlikeInformationCenter.NetworkandLenovo.Company.TheperiodcanalsobereplacedbyaperiodinChinese.Theentiresystemiscompatiblewiththeexistingdomainnamesystem,andsupportscompleteinteroperabilityanalysisinsimplifiedandtraditionalChinese.Inaddition,aftertheregistrationoftheChinesedomainname,itcanalsosupportapplicationservicessuchasChinesee-mailaddressfunctionandChinesevirtualhost.
ComparisonofChineseURLsandChineseDomainNames
ChineseURLsandChinesedomainnamesarebothsolutionstosolvetheChinesepeople’sunsuitabilitytoexpressinEnglishontheInternet.Thetwolooksimilar,buttheyarequitedifferentinnature.Firstofall,theChinesewebsiteisbasedonthedomainname.Forexample,ifyouwanttotakeataxitoCCTV,youcantellthedriveritshousenumber,whichisequivalenttoadomainname,oryoucandirectlytellthedriver"I'mgoingtotheTVstation",whichislikeawebsite.Butitisundeniablethatthehousenumberisthebasis,andthe"televisionstation"directlymentionedisanickname.Secondly,intermsofthehierarchicalstructure,theChineseURLisaparallelstructure,andthereisnoconceptualdistinctionbetweendomainsandsubdomains;whileChinesedomainnamesmaintainthehierarchicalcharacteristicsofdomainnames.AlargeorganizationcanestablishaChinesedomainnamesystemforitsvariousdepartments,suchas:Acertainmajor.Acertaindepartment.Acertainuniversity.China.Finally,Chinesedomainnamescansupportotherapplicationservices,suchasE-mail,etc.,whileChineseURLsaresuitableforintelligentsearch.
Structureoperation
TheInternetdomainnamesystemisatreestructurewiththefollowingforms:com(enterprise),net(networkoperationserviceagency),gov(governmentagency),org(Non-profitorganizations)andedu(education)domainsaremanagedbyInterNic,andtheirregistrationandoperationareinchargeofNetworkSolution.
Newtop-leveldomainnames
Thesevennewtop-leveldomainnamesare:firm(corporateenterprise),shop(store),web(entitythatwantstohighlightWorldWideWebactivities),arts(mainEntitiesengagedinentertainmentandculturalactivities),rec(mainlyengagedinentertainmentandculturalentities),info(mainlyengagedininformationserviceentities),nom(somepeoplewhowishtopublishpersonalinformationontheInternet)willbelaunchedin1998.ThesedomainnameregistrationservicesItisundertakenbyanumberofinstitutions,andCNNICisalsofortunatetobeoneoftheregistrationinstitutions;thenationaldomainnamesformulatedinaccordancewiththeISO-3166standardaregenerallyoperatedbytheNIC(NetworkInformationCenter,NetworkInformationCenter)ofeachcountry.mycountry'sdomainnamesystemisdividedintotwosetsofcategorydomainnamesandadministrativeareanames.
Categorydomainname
Categorydomainnamereferstothefirstsixinthebottomlineofthefigure.
Domainnamesaredividedinto:AC-scientificresearchinstitutions;COM,TOP-industry,commerce,financeandothermajors;EDU-educationalinstitutions;GOV-governmentdepartments;NET-Internet,Informationcenterandoperationcenterconnectedtothenetwork;ORG-variousnon-profitorganizations.
Administrativeareanames
Administrativeareanamesaredividedaccordingtothevariousadministrativeareasinourcountry,andtheirclassificationstandardsaredeterminedaccordingtothenationalstandardsissuedbytheStateTechnicalSupervisionBureau,including"administrativeareanames"34One,applicabletoallprovinces,autonomousregions,andmunicipalitiesinChina,namely:BJ-Beijing;SH-Shanghai;TJ-Tianjin;CQ-Chongqing;HE-HebeiProvince;SX-ShanxiProvince;NM-InnerMongoliaAutonomousRegion;LN-LiaoningProvince;JL-JilinProvince;HL-HeilongjiangProvince;JS-JiangsuProvince;ZJ-ZhejiangProvince;AH-Anhui;FJ-FujianProvince;JX-JiangxiProvince;SD-ShandongProvince;HA-HenanProvince;HB-HubeiProvince;HN-HunanProvince;GD-GuangdongProvince;GX-GuangxiZhuangAutonomousRegion;HI-HainanProvince;SC-SichuanProvince;GZ-GuizhouProvince;YN-YunnanProvince;XZ-TibetAutonomousRegion;SN-ShaanxiProvince;GS-GansuProvince;QH-QinghaiProvince;NX-NingxiaHuiAutonomousRegion;XJ-XinjiangUygurAutonomousRegion;TW-Taiwan;HK-HongKong;MO-Macau.
CNdomainnamesareoperatedbyCNNICexceptedupointcnwhichisoperatedbyCernNic(educationnetwork).
Forms
Thereareusuallytwoformsofdomainnameservers:domainnameserversandcachedomainnameservers.
DomainNameServer
Adomainnameserverisrequiredinthefollowingsituations:youwanttoprovideDNSinformationtotheworldandgiverequests.Registeredadomainsimilartoexampleorg,andyouneedtoassigntheIPtothehostnameunderit.AcertainIPaddressblockrequiresareverseDNSentry(IPtohostname).Thebackupserver,ortheslaveserverasitisoftencalled,willrespondtoqueryrequestswhenthemasterserverhasproblemsorcannotbeaccessed.
CacheDomainNameServer
Thefollowingsituationsrequireacachedomainnameserver:ThelocalDNSservercancacheandgetaresponsefasterthandirectrequeststoexternaldomainnameservers.
Whensomeonequerieswww.FreeBSDorg,theresolverusuallysendsarequesttothedomainnameserverofthesuperiorISPandgetsaresponse.IfthereisalocalcachingDNSserver,thequerywillonlybesenttotheoutsideworldbythecachingDNSserverforthefirsttime.OtherquerieswillnotbesentoutsidetheLANbecausetheyalreadyhavealocalcache.
Domainnameresolution
Overview
Justlikewhenwevisitapersonforthefirsttime,weneedtoknowthehousenumberoftheperson’shouse,andthenlookforitaccordingtotheaddress.ItisnotenoughtoknowthedomainnameofacertainmachineontheInternet.Theremustbeawaytofindthatmachine.ThetaskoffindingthismachineiscompletedbyadevicecalledadomainnameserverontheInternet,andtheprocessofcompletingthistaskiscalleddomainnameresolution.
Process
WhenamachineasendsadomainnameresolutionrequesttoitsdomainnameserverA,ifAcanresolveit,itwillsendtheresolutionresulttoa,otherwise,AwillsendittoitTheupper-leveldomainnameserverBsendsaresolutionrequest.IfBcanresolve,theresolutionresultissenttoa.IfBcannotresolve,therequestissenttotheupper-leveldomainnameserverC,andsoon,untiltheresolutioniscomplete.
Server
ThefullnameofDNSisDomainNameServer,aprogramthatsavesadomainname(domainname)andthecorrespondingIPaddress(IPaddress)Tabletoresolvethedomainnameofthemessage.AdomainnameisthenameofacomputerorcomputergroupontheInternet.Itisusedtoidentifythecomputer'selectroniclocation(sometimesalsoreferredtoasgeographiclocation)duringdatatransmission.Adomainnameiscomposedofaseriesofnamesseparatedbydots.Itusuallycontainsthenameoftheorganizationandalwaysincludesatwotothreelettersuffixtoindicatethetypeoforganizationorthecountryorregioninwhichthedomainislocated.
ThereasonwhydomainnameresolutiondoesnottakealongtimeisbecauseInternetaccessproviders,suchasBeijingTelecomandHenanTelecomthroughSunshineDNS,areusedtospeedupusers’accesstowebpages.DNSrecordsofmanydomainnamesarecachedintheDNSserver,andthennetworkallocationismadeaccordingtotheuser'slocation.Inthisway,whenauserofthisaccessproviderwantstoopenacertainwebpage,theaccessprovider’sserverdoesnotneedtoquerythedomainnamedatabase,butdirectlyusestheDNSrecordsinitscache,thusspeedinguptheuser’saccesstothewebsite.Thisisitsgreatestadvantage.
ThedisadvantageisthatthecacheoftheInternetaccessprovider’sISPwillbestoredforaperiodoftimeandwillonlybeupdatedwhenneeded,andthereisnostandardforthefrequencyofupdates.SomeISPsmayupdateonceanhour,andsomemayonlyupdateonceadayortwo.
Sothenewlyregistereddomainnameisgenerallyfastertoresolve.BecauseallISPsdonothavecaches,ISPshavetoquerythedomainnamedatabasetoobtainthelatestDNSdatawhenusersvisit.
IftheDNSrecordoftheolddomainnameischanged,thecacheddataofISPsaroundtheworldisnotupdatedimmediately.Inthisway,fordifferentusersunderdifferentISPs,somecanobtainnewDNSrecordsrelativelyquickly,andsomehavetowaitforthenextupdateoftheISPcache.