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Chapter 7

DNSContents

Need of DNS Origin of DNS Understanding DNS Hierarchy of DNS Components of DNS Working of DNS

Objectives

After completion of this module you will be able to know !he need of DNS Origin of DNS Working method of DNS Hierarchy of DNS Components of DNS

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DNSWhile DNS is one of the least necessary technologies that make up the )nternet as weknow it( it is also true that the )nternet would ne*er ha*e become as popular as it is todayif DNS did not e+ist, !hough this may sound like a bit of a contradiction( it is true( nonethe less,

DNS stands for two things Domain Name Ser*ice -or Domain Name System. andDomain Name Ser*ers, One acronym defines the protocol/ the other defines the machinesthat pro*ide the ser*ice, !he 0ob that DNS performs is *ery simple it takes the )$addresses that computers connected to the )nternet use to communicate with each otherand it maps them to hostnames,

Sounds pretty simple( doesn1t it2 Well( it is, 3ut 0ust because it1s simple doesn1t make itany less important,

Human beings tend to ha*e a difficult time remembering long strings of seeminglyarbitrary numbers, !he way that our brains work( it1s difficult to make information likethat stick, And that is where DNS comes in, )t allows us to substitute words or phrases forthose strings of numbers, Words are a lot easier for people to remember than numbers(especially when they can be tied to a specific idea that is linked to the website,

3ut how does DNS work2 What makes it operate2 How did it start2

7.1 Web site address

3efore we get into DNS( let4s start off with breaking down a web address, )t essentiallygi*es where the web page is( and how you need to talk to it, 5ets use the e+ample of

http 66www,bsnl,co,in6pages6cellone,htm

!he first part is 7http 667( and that tells your $C what protocol -what language so to speak.to use talking with this site, )n this case( you are using H!!$ -Hyper!e+t !ransfer$rotocol., Another *ery common one for web designers to use is 7ftp 667 or 8ile !ransfer$rotocol, 9ou would use it to connect to your web ser*er to put the web pages youcreated onto the ser*er, 9ou also see 7https 667 :uite commonly, !his simply means thatthe connection between you and the web ser*er is secure -meaning the information being

sent back and forth is encrypted., 9ou should see 7https 667 when you are checking out(especially when they are entering credit card information,

!he ne+t part( 7www,bsnl,co,in7 is called the Domain Name , !he 7www7 used to be moresignificant than it is today, !oday( the 7www7 is( for the most part( assumed and you canget to the same page regardless of whether or not you type in 7www7 your browser,

!he part 76pages6cellone,htm7 tells the web ser*er to look in the directory called 7pages7

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and send the file called 7cellone,htm7 to your browser, )t is 0ust like the directories onyour $C,

!he nter DNS, DNS is an acronym for 7Domain Name Ser*ice7, )t1s whole purpose in life isto translate between the friendly 7www,bsnl,co,in7 and the notEsoEfriendly #@,@,@ ;,"?, )thandles this translation for web sites( email( 8!$ ser*ers( database ser*ers( or anymachine within a domain name, 5et1s dig into the process of how that works,

DNS means Domain Name Ser*ice, )t is actually a ser*ice that can keep large number ofmachines4 )$ addresses for huge network communication, Now the :uestion arises why isthis needed, 5et4s understand this with the help of an illustration,

Example: 5et4s say rose@( rose ( rose ( rose?( and rose are the machines in a network(then for communication between each machine( each machine4s 6etc6hosts in Uni+ -orhosts,t+t in Windows. file should ha*e all the fi*e entries of the machine name, Withinthis small network there would be no problem if you add another machine say rose# inthe network, 3ut for this too( the network administrator has to go to each machine( addthe rose# in 6etc6hosts file and then comeback to the new comer rose# machine and addall the other entries -rose@,,,rose . including its own name also in 6etc6hosts -or hosts,t+t.file,

3ut what if the network is setup with say #B machines and a #@st machine has to beadded2 !hen administrator will ha*e to go to each machine again and write the newmachine4s name at 6etc6hosts6 -or hosts,t+t. file and again comeback and write all the #Bmachines name on the #@st machine4s etc6hosts file which is a tedious and time taking

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0ob, !hus( it is better to keep a centraliFed ser*er( where all the )$ addresses will stay andif a new one does enter into the network then the change will ha*e to be done at theser*er and not on the client4s machine,

7.4 The Origin of DNS

5ike almost e*erything else originally associated with the )nternet( DNS traces its originsto A%$AN>!, Alphabetic hostnames were introduced shortly after its inception as ameans of allowing users greater functionality( since the numeric addresses pro*eddifficult to remember,

Originally( e*ery site connected to A%$AN>! maintained a file called GHOS!S,! !1which contained the mapping information for all of the numeric addresses used there,!hat information was shared through A%$AN>!, Unfortunately( there were many

problems that arose from that setup, >rrors were commonplace and it was inefficient tomake changes considering they needed to be made on each and e*ery copy of the

HOS!S,! ! file,

3y No*ember of @I" ( a plan was laid out in %8Cs ""@( "" ( and "" ( also known asG!he Domain Names $lan and Schedule(1 GDomain Names EE Concepts And 8acilities(1and GDomain Names EE )mplementation And Specification,1 !hese three %8Cs definedwhat has de*eloped into DNS as we know it today, Surprisingly( not a whole lot haschanged since that time,

7.5 nderstanding DNS

DNS organiFes groups of computers into domains , !hese domains are organiFed into ahierarchical structure( which can be defined on an )nternetEwide basis for public networksor on an enterpriseEwide basis for pri*ate networks -also known as intranets ande+tranets., !he *arious le*els within the hierarchy identify indi*idual computers(organiFational domains( and topEle*el domains, 8or the fully :ualified host nameomega,microsoft,com( omega represents the host name for an indi*idual computer(microsoft is the organiFational domain( and com is the topEle*el domain,

!opEle*el domains are at the root of the DNS hierarchy and are therefore also called rootdomains , !hese domains are organiFed geographically( by organiFation type( and byfunction, Normal domains( such as microsoft,com( are also referred to as parent domains ,!hey4re called parent domains because they4re the parents of an organiFational structure,$arent domains can be di*ided into subEdomains( which can be used for groups ordepartments within an organiFation,

!here are three types of !5Ds, !hey are

@, &eneric or OrganiFation based !5D -e,g com( edu( go*( mil( net( org( int( aero(museum( etc.

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, &eographical or country based !5D -e,g, in( us( au( etc., !his !5S is ha*ing letters,

, )n*erse -e,g, arpa., !his !5D is to find domain name from )$ address,

SubEdomains are often referred to as child domains , 8or e+ample( the fully :ualifieddomain name -8JDN. for a computer within a human resources group could bedesignated as 0acob,hr,microsoft,com, Here( jacob is the host name( hr is the childdomain( and microsoft.com is the parent domain,

Domain Name System -DNS. is an )nternet ser*ice that translates domain names into )$addresses, DNS pro*ides a database that stores a list of host names and theircorresponding )$ address, !his process is called name resolution or mapping, Nameresolution occurs when a program on a local computer re:uests a remote host forresources, !he local computer sends the host name of the ser*er as part of the re:uest, 3yusing the host name as an inde+( the DNS database is searched to resol*e the )$ address

of the host,

7.! Do"ain Na"e S#a$e %ierar$hy

DNS is organiFed in a hierarchical tree structure, >ach branch in the tree represents adomain and each subEbranch in the tree represents a subEdomain, DNS consists ofmultiple le*els of domains, !he domains are identified based on the le*el at which theyare placed in the hierarchical tree structure, !he *arious le*els of domains in a domainname space hierarchy are

Domain root: This is the node at the highest point of the hierarchical DNS

tree. In a DNS domain name, a trailing period represents the domain roottree (.). It is also shown as two empty quotation mar s representing a null!alue.

Top"le!el domain: This is the ne#t le!el in the hierarchical tree structure. Itrepresents the region or the type of organi$ation to which a domain %elongs.& top"le!el domain name contains two or three letters such as com, edu, andmil.

Second"le!el domain: This is a domain name registered under a specific top"le!el domain, such as organi$ations %ased on type and geographical locationsThe Second"le!el domain names ha!e names with !aria%le length. 'or

e#ample, e#ample.com is a second"le!el domain name.

Su%domain: This is a domain created under a second"le!el domain.rgani$ations need to create additional domains to represent organi$ationalhierarchy and !arious functional groups. & second"le!el domain alsocontains a name with !aria%le length.

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ost or resource: & host or resource computer is the last in the DNShierarchy. It helps find the I* address of the computer %ased on its hostname.

7.7 &o"#onents of DNS

!he building blocks of DNS are the domain namespace( resource records( DNS ser*er(and DNS clients, 8igure ;E@ lists the *arious components of the DNS

8igure ;E@ Karious Components of the DNS

7.7.1 Domain Namespace

A DNS domain is a logical group of computers that either re:uest for DNS ser*ice orrespond to a ser*ice re:uest, Howe*er( this logical group might also represent the

physical network, A DNS domain can represent all the computers internetworked in asmall business network, At the same time( DNS domain can also comprise physicalnetwork that is spread across geographical locations, !his logical grouping of computersnetwork is further grouped into smaller administrati*e units( called administrati*edomains, An administrati*e domain is a group of computers in a single administrati*eunit, >ach administrati*e domain has two or more name ser*ers for name resolution, Alladministrati*e domains registered with the )nternet form a hierarchical structure( calledthe DNS domain namespace,

!he domain namespace follows a hierarchical tree structure, >ach node and leaf on thetree represents either a set of resource or a DNS host, 3ased on the position in thenamespace hierarchy( each node is assigned a label, !he root at the top of the hierarchy isassigned the null label and is called the root domain, !he nodes below the root are calledthe topEle*el domains, !he nodes below the topEle*el domain are called secondEle*eldomain, An e+ample of a secondEle*el domain is e+ample,com where Gcom4 is the topE

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le*el domain, Domain created under an e+isting domain node is called a subdomain, 8ore+ample( resource,e+ample,com is a subdomain of e+ample,com, !he name of asubdomain is followed by the name of the domain that contains it, )n a namespace(domain name are read from left to right, >ach label in a domain name is separated by adot -7,7., A complete domain name also includes the root label ends with a dot,

8igure ;E shows the domain namespace hierarchy

8igure ;E Domain Namespace Hierarchy

When an organiFation registers for the secondEle*el domain( a topEle*el domain label isassigned based on the type of organiFation, !able ;E@ lists the commonly uses topEle*eldomains

!able ;E@ Commonly Uses !opE5e*el Domains

Top"+e!elDomain

Description

arpa Used by resources that belongs to the Ad*anced %esearch $ro0ectAgency -A%$A.,

com Used by businesses that uses the )nternet for the commercial purposes,

edu Used by schools( colleges( and uni*ersities,

go* Used by all types of go*ernment organiFations,

int %eser*ed for international usage,

mil Used by all types of military organiFations( such as Department ofDefense -DoD.,

net Used by )nternet and telephone ser*ice pro*iders,

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!able ;E@ Commonly Uses !opE5e*el Domains

Top"+e!elDomain

Description

org Used by charitable institutions,

biF Used by businesses,

name Used for registration by indi*iduals,

info Offers unrestricted use,

Note Apart from these topEle*el domains( country region codes( such as uk( are used incon0unction with the listed topEle*el domain names,

7.7.2 Resource Records

%esource %ecords -%%s. store and map domain names to the type of resources storedwithin a domain, >ach node in the hierarchical tree is associated with a set of resourceinformation,

%esource records contain information( such as the type( class( !!5( and %DA!A, !heowner information is not maintained because it is *ery implicit to a resource record, !he*ariable part of the resource records maintained in a domain is the %DA!A, !hisdifferentiates between the resource records,

Note Short !!5s should be used to reduce caching in the resol*er, !o prohibit caching( aFero *alue can be assigned to the !!5,

7.7.3 DNS Server

DNS ser*ers( also called as name ser*ers( are responsible for name resolution in adomain, >ach domain normally has two or more DNS name ser*ers, !he domainEspecificinformation( such as the list of )$ addresses along with their host names( is stored in adistributed database called the domain database, !his information is distributed across thename ser*ers a*ailable in the domain, Name ser*ers use this information to process:ueries recei*ed from a DNS client, >ach DNS ser*er is responsible for a specific part ofthe domain database, !he DNS ser*er becomes authoritati*e for that part of the database,As an output( name ser*er either sends back the )$ address of the desired host or sendsreferral that closely match to the address, Howe*er( the entire domain database is

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replicated among the name ser*ers to help name ser*er continue the name resolution process in case of communication link failure or inaccessibility of DNS hosts,

!he *arious types of name ser*ers are

$rimary ser*er Stores the master copy of the domainEspecific information,Changes in the domainEspecific information are updated on the primary nameser*er, As per the DNS design specification( each administrati*e domain shouldha*e two authoritati*e name ser*ers, One of the authoritati*e name ser*ers isdesignated as the primary ser*er,

)t stores the DNS database for its Fone of authority, )t is responsible for answering a:uery from client, )t is an authenticated ser*er/ hence called as Authoritati*e Se*er

Secondary ser*er Stores the copy of the master data file stored in the primaryname ser*er, >ach domain has one or more secondary name ser*ers, A secondary

name se*er is also authoritati*e for a domain, Secondary name ser*ers aredelegated authority by the primary name ser*er to perform name resolution, !hesecondary name se*er are immediately updated in case of a change in the masterdata file,

)t stores a copy of the database of the $rimary ser*er, $eriodically it will collect thedatabase information from the $rimary ser*er, )t is also an authenticated ser*er hencecalled as Authoritati*e Se*er,

)f primary ser*er fails( then this will answer the :uery, Once in hours -normally.( it will be updated automatically,

CacheEonly ser*er Stores the information recei*ed by the name ser*ers in thememory till it e+pires, !his cached information is used to resol*e :ueries, Acaching ser*er that is not authoritati*e for a domain is called a cacheEonly ser*er,Howe*er( all name ser*ers are caching ser*ers,

!o a*oid response delay for a :uery( Cache ser*er is used, )t is a NonEauthority ser*er,!he !!5 -!ime !o 5i*e. parameter is related to this ser*er, 8or e+ample !!5 durationcan be a ma+imum of days,

7.7.4 DNS Client

DNS clients are local computers that are configured to recei*e DNS ser*ices from a DNSser*er, DNS clients are configured with a resol*er that :ueries DNS ser*ers, !he resol*erin a DNS client works as an interface between the applications installed on the DNSclient and the DNS ser*er, %esol*er recei*es re:uests from applications such as email

programs and sends a :uery to the DNS ser*er, After the DNS ser*er resol*es the :ueryusing the resource records( the desired information is returned to the DNS client in a data

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format that is compatible with the local computer, !o resol*e a :uery( DNS client eitherconsults se*eral DNS ser*ers or retrie*es the information from local cache,

DNS client and enduser program reside on a single computer, !he interface that works between a DNS client and enduser program is dependent on the local DNS ser*er,

!he functions of a DNS client are

Name to address translation DNS client translates userEfriendly domain names to)$ addresses,

Address to name translation DNS client also translates )$ addresses to userEfriendly computer name,

&eneral lookup function DNS clients help enduser programs to retrie*e arbitraryinformation from a DNS ser*er, )nstead of :uerying a DNS ser*er against )$addresses or userEfriendly name( DNS client can also re:uest information

mapping to the specified type or class of a resource records,

7.' Na"e S#a$e %ierar$hy on the Internet

!he DNS name space hierarchy for the )nternet contains two e+tra le*els of domains thana local domain name space hierarchy, !hese two e+tra le*els form the topmost layers ofthe )nternet name space hierarchy are called the root domain, )n the name space( the rootdomain is represented by a dot -,., !he root domain contains two types of subdomains(organiFational and geographical, !hese subdomains are called topEle*el domains, !heorganiFational topEle*el domains are com( net( org( mil( go*( edu( and int, !he geographictopEle*el domains determine the location of domains and are assigned a twoElettered or a

threeElettered word, 8or e+ample( subdomains in 3ritain contain uk as a part of theirdomain name, 8igure ;E shows the name space hierarchies of domains on the )nternet

8igure ;E Name Space Hierarchies on the )nternet

!he go*erning bodies of the )nternet maintain the )nternet root domain( topEle*elorganiFational and geographic domains, An organiFation needs to apply for membershipto 0oin the )nternet under the organiFational or the geographical hierarchy,

7.( %o) DNS Wor*s?

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)n a nutshell( DNS translates )$ addresses into hostnames and back again, !he hostnamesare for the benefit of human end users, !he )$ addresses are the only essential thing( asfar as the computers are concerned, )n a longer form( we need to begin by looking at thedifferent types of DNS ser*ers,

!he first type of ser*er is called a G%oot Name Ser*er,1 >ach !op 5e*el Domain -suchas ,com( ,edu( ,us( ,in( ,sg etc. has one or more %oot Name Ser*ers which are responsiblefor determining where the indi*idual records are held, !hese ser*ers are fairly static ande*ery machine on the internet has the capability of reaching any of them( as needed,

!he ser*ers that the %oot Name Ser*ers direct :ueries to are called GAuthoritati*e NameSer*ers1, !hese are the ser*ers which hold the actual information on an indi*idualdomain, !his information is stored in a file called a GLone 8ile,1 Lone files are theupdated *ersions of the original HOS!S,! ! file,

!he final type of name ser*er is called a G%esol*ing Name Ser*er1, !hese are the ser*ers

that do the ma0ority of the work when you are trying to get to a machine with a certainhost name, 3esides being responsible for looking up data( they also temporarily store thedata for hostnames that they ha*e searched out in a cache( which allows them to speed upthe resolution for hostnames that are fre:uently *isited,

!he manner in which these ser*ers work together is fairly straightforward, When youattempt to go to a website( you type in a hostname in your web browser, 5et1s say( forcon*enience( that you are going to www,foo,org, )n your computers1 settings is a list ofresol*ing name ser*ers which it :ueries to find out what www,foo,org1s )$ address is,

!he first thing that the resol*ing name ser*ers will do is check their caches to see if the

DNS information for www,foo,org is already there, )f it isn1t( they will go and check withthe ,org root name ser*er to see which authoritati*e name ser*er holds the Fone file forfoo,org, Once they ha*e that ser*er1s )$ address( they connect to it,

Once the resol*ing name ser*er has :ueried the authoritati*e name ser*er( it replies backto your computer with one of a number of different things, )deally( it will report backwith the correct )$ address and allow your computer to connect to the web ser*er andshow you the web page that you were looking for, Howe*er( if the authoritati*e ser*er isdown( doesn1t ha*e a record for the specific hostname that you are looking up( or if theroot ser*er doesn1t ha*e a record that the domain name e*en e+ists( the resol*ing nameser*er will report an error to your computer,

Example:

5et1s use the e+ample that %am types 7www,bsnl,co,in7 into his web browser, How doeshis $C find the web ser*er that has the page he is looking for( among the thousands ofweb ser*ers out there2

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@, %am types in www,bsnl,co,in to his browser,

, %am1s $C looks at it1s configuration, )t will find something called 7DNS Ser*er7or 7name ser*er7 and there will be an )$ address associated with that, 5et1s say itis @I",#,@,@, %am1s $C sends a message to @I",#,@,@ and asks 7) am looking for

the )$ address of www,bsnl,co,in( can you tell me what it is27

, !he DNS Ser*er -@I",#,@,@. gets the message( and assuming that the ser*eralready knows what the )$ address of www,bsnl,co,in is( it tells %am1s $C that the)$ address is #@,@,@ ;,"?,

?, %am1s $C gets the message that the )$ address of www,bsnl,co,in is #@,@,@ ;,"?,So his $C sends a message to #@,@,@ ;,"? and asks 7send me the default web pageat #@,@,@ ;,"?7,

, !he web ser*er -whose )$ address is #@,@,@ ;,"?. sends the web page to %am4s browser,

!hat is a simplistic e+ample of how your $C finds a particular webEser*er and web page,!he process of matching a domain name to a )$ address is called resol*ing, So your $Cresol*es the )$ address from the domain name, 5et1s get into a little more detail,

8or step ( how does %am1s $C know that the )$ address of the DNS Ser*er is2 !here are ways it learns what the address is, !he first is that %am asked his )S$ what the addresswas( and entered it himself, !here are times manually entering -also known as staticallyentering. the address is necessary or desirable( but usually the )S$ automatically tellsyour $C what the )$ address of the DNS ser*er is, !his process is called 7DHC$7 orDynamic Host Configuration $rotocol, When you select 7Obtain )$ addressautomatically7 in your Windows Network connections page( you are telling your $C touse DHC$ and to ask the )S$ to gi*e you the DNS Ser*er address -among a bunch ofother things.,

)n step ( we assumed that the DNS ser*er already knew what the )$ address of

www,bsnl,co,in was, What if it didn1t already know2 5et1s assume that the DNS ser*er%am1s $C sent a re:uest to( doesn1t know where www,bsnl,co,in is,

Ha*e you e*er noticed that there are only so many *ariations of the end of the domainname2 !here are ,com( ,go*( ,net( ,org( ,us( ,in( ,biF( among others, When a DNS ser*errecei*es a re:uest to resol*e an )$ address -translate from a domain name to a )$ address.for a domain that it doesn1t know the answer to( it sends a message to any one of a smallnumber of ser*ers, !hat small number of ser*ers are responsible for knowing what the

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7authoritati*e ser*er7 is for >K>%9 domain name, A realm would be ,com( or ,org fore+ample( and is properly called a topEle*el domain,

What is an authoritati*e ser*er2 An authoritati*e ser*er is a DNS ser*er that has aStatement of Authority configured for a particular domain name, !hat means that the

ser*er has absolute and total knowledge of the domain( any information that contradictsthe information that the ser*er has is wrong( it is the final word, !his becomes moreimportant a little later, 8or purposes of this discussion( let1s ignore backup authoritati*eser*ers,

!he message that %am1s DNS ser*er sends to the topEle*el domain ser*er 7what is theauthoritati*e ser*er for bsnl,co,in27, )t is important to understand( that %am1s DNS Ser*eris NO! asking 7what is the )$ address of the web ser*er for only one creations27, )t isonly asking 7where do ) go to find out where the web ser*er for bsnl,co,in is27

Once %am1s DNS ser*er knows where to go to get the answer for %am1s re:uest( it sends

a message to the authoritati*e ser*er asking 7what is the )$ address of the web ser*er for bsnl,co,in27, !he authoritati*e ser*er responds( and %am1s DNS Ser*er tells %am1s $C the)$ address it needs to connect %am to the webpage he is looking for,

!o summariFe the past few paragraphs( %am1s DNS ser*er recei*es a re:uest for an )$address that it doesn1t know, !hat ser*er makes a re:uest of a topEle*el domain ser*er(and gets a response with where to go to get the information that %am is re:uesting, !heDNS ser*er then makes a re:uest of the authoritati*e ser*er( and forwards the answer itrecei*es to the $C that made the first re:uest, )t sounds long and comple+( but it happens*ery :uickly, One way to speed up the process is called caching, Caching is where theDNS ser*er remembers the response from the authoritati*e ser*er for a period of time, So

if 3abu makes the same re:uest minutes after %am did( the DNS ser*er doesn1t ha*e torepeat the whole process, Caching will be brought up again in a bit,

%emember that %am1s DNS ser*er cached the address for the web ser*er ofwww,bsnl,co,in( meaning that it remembers that www,bsnl,co,in is has the )$ address of#@,@,@ ;,"?, 'ost DNS ser*ers are set to remember that information for ? hours, So if%am re:uests your web page at noon on 'onday( %am1s DNS ser*er will cache the )$address of your web ser*er until noon on !uesday, )f you change hosts at @pm on'onday( %am will get your old website until at least noon on !uesday, His DNS ser*er isgi*ing %am1s $C the information it remembers( it doesn1t check to see if that is stillaccurate, So if your old website is down -maybe you ha*e mo*ed hosts( for e+ample.(%am can1t get to your new website( until his DNS ser*er refreshes the information -whichwill point to the new web site.,

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