Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 461Chapter 23 – Domain Name System (DNS)23.2 Names for MachinesIt is usually easier to refer to a machine as juniper than as 138.26.66.623.3 Flat Namespace► Early ARPAnet: unique names for machines, registered with Network Information Center► Continued until 1000 machines► Beyond that a massive bureaucracy would have been necessary to ensure uniqueness of names► The Domain Name System was created2Chapter 23 – Domain Name System (DNS) - continued23.4 Hierarchical NamesCity “Athens” – Athens, AL or Athens, GA or Athens, TN … ?Specifying state removes ambiguity.23.5 Delegation of Authority for NamesIf you want to start a new city and call it “Athens,” you probably need permission from the state – you will probably be allowed to do it if there is not already and “Athens” in your state.Once you have permission for the city name, you probably have the authority to name the neighborhoods.In DNS “The topmost level of the hierarchy divides the namespace and delegates authority for each division; it need not be bothered by changes within a division.”323.6 Subset AuthorityThis can be carried down to more levels:local.group.siteThere is an analogy to the telephone numbering system:<area code> – <exchange> – <extension> (although in this case the highest level is written on the left).4In the Internet, hierarchical machine names are assigned according to the structure of organizations that obtain authority for parts of the namespace, not necessarily according to the structure of the physical network connections.Example 1:Two small departments, philosophy and foreign languages, might share a floor in the Humanities building; their domain names, pl.uab.edu and fl.uab.edu are different but they may share a single LAN and therefore have the same IP (sub)network address.Example 2:The CIS department has a large number of hosts and several LANs. All hosts have domain names with the same suffix, cis.uab.edu but their IP addresses are 138.26.64.x 138.26.65.y 138.26.66.z523.7 Internet Domain NamesThe Domain Name System has two, conceptually independent aspects:■ (“abstract”) - name syntaxand delegating authority over names■ (“concrete”) - distributed computing system that efficiently maps names to addressesSection 23.7 continues with the first topic.623.7 Internet Domain Names – continuedDomain namecs.purdue.educontains three labels. This is the lowest-level domain or third-level domain.Any suffix of a label in a domain name is also called a domain:purdue.edu is the second-level domainedu is the top-level domain(labels can be up to 63 ASCII characters long; the total length of the name can be up to 255 characters long)723.8 Top-Level DomainsFigure 23.1 Top-level domains823.8 Top-Level Domains - continuedThe domain name space can be represented by a rooted tree:top-level domainsecond-level domainthird-level domainDifferent from Comer! More latermerlin Figure 23.2923.8 Top-Level Domains - continuedtop-level domainsecond-level domainthird-level domainAll domain names end in .e.g. cs.purdue.edu. but the . is usually omitted in speech (not omitted in coding).The domain name of a subtree is a list of its labels, separated by periods, along the path to the root.1023.8 Top-Level Domains - continuedConceptually, the top-level names permit two independent naming hierarchies:■ geographic■ organizationalAn organization can choose which way it wants to be known.In the US we mostly see the organizational structuree.g uab.edu not uab.al.usOther countries often use the geographic hierarchye.g. bham.ac.uk for the University of Birmingham (England).For the geographic scheme each country has a two-letter codee.g. us for United States Beneath this top level there are second-level domains for each state,e.g. al.us1123.8 Top-Level Domains - continuedThere is a registrar for each top-level domain,e.g. eduAn organization applies to the top-level registrar for approval of a second-level domain,e.g. uab.eduA department of an organization applies to the organization administration for approval of a third-level domain,e.g. cis.uab.eduThe departmental administrator may assign fourth-level domains,e.g. juniper.cis.uab.edu1223.9 Name Syntax and TypeA record in the domain name system specifies the domain name, and also the type of this particular record, e.g.■ IP address of the named machine (type A)■ mailbox (type MX – name of the machine running the SMTP server)At first we’ll use the first type for examples – other types later1323.9 Name Syntax and Type - continuedThe syntax of a domain name does not determine what type of object is represented.gwen.purdue.educould be an individual machine (leaf of tree), althoughcs.purdue.eduIs a non-leaf domain, with numerous machines under it.14gwenOne cannot distinguish the names of subdomains from the names of individual objects or the type of an object using only the domain name syntax.Comer’s example: machine gwen.purdue.edu.1523.10 Mapping Domain Names to AddressesRecall that:DNS has two, conceptually independent aspects:■ (abstract) - name syntaxdelegating authority over names■ (“concrete”) - a distributed computing systemthat efficiently maps names to addressesNow we consider the second aspect.1623.10 Mapping Domain Names to Addresses – continuedThe domain name system includes an■ efficient■ reliable■ general-purpose■ distributedsystem for mapping names to addresses.1723.10 Mapping Domain Names to Addresses – continuedWhat is meant by these characteristics?■ efficient most names can be mapped locally, without needing Internet traffic (probably not true of home network)■ reliable no single machine failure will prevent the system from operating correctly■ general-purpose not restricted to mapping machine domain names to IP addresses (but we will often use this as our example)■ distributed a set of servers at multiple sites cooperatively solve the mapping problem18Name ServerThis is a server that supplies name-to-address translation, mapping from domain names to IP addresses (the