Domain Name System (DNS)

DNS, or Domain Name System, is a hierarchical system for identifying hosts on the Internet or on a private, corporate TCP/IP internetwork.

What is Domain Name System (DNS)?

DNS is a hierarchical system for identifying hosts on the Internet or on a private, corporate TCP/IP internetwork. The Domain Name System (DNS) provides:

  • A method for identifying hosts with friendly names instead of IP addresses
  • A distributed mechanism for storing and maintaining lists of names and IP addresses of hosts
  • A method for locating hosts by resolving their names into their associated IP addresses so that network communication can be initiated with the host

How It Works

The DNS namespace is hierarchical in structure, beginning with the root domain, which branches to top-level domains, then second-level domains, and so on to the individual host name.

For example, the fully qualified domain name (FQDN) can be broken down as follows:

  • Host name: barney
  • Third-level domain: northwind (stands for Northwind Traders Ltd., a fictitious Microsoft subsidiary)
  • Second-level domain: microsoft (Microsoft Corporation)
  • Top-level domain: com (commercial domain)

The root domain has a null label and is not expressed in the FQDN.

Graphic D-36. Domain Name System (DNS).

The DNS is implemented as a distributed database using name servers located at various points on the Internet. Clients called resolvers can perform name lookups by contacting these name servers, which resolve host names into IP addresses. In Microsoft Windows NT and UNIX BIND name servers, the DNS database of host name to IP address mappings must be created manually by entering resource records for each host that needs to be resolved. In Windows 2000, the new dynamic update can be used to register host names automatically, and zone information can be stored and replicated using Active Directory.

Each name server on the Internet is responsible for a subset of the DNS namespace known as a zone of authority. Each zone of authority can consist of one or more domains and subdomains. The most important name servers on the Internet are the dozen or so root name servers, which are responsible for maintaining the infrastructure of the domain name system. These root name servers are maintained mostly by the Internet Network Information Center (InterNIC) and by U.S. military agencies (because the Internet evolved from the ARPANET project of the U.S. Defense Department in the 1970s).

A name server can function in one of four roles in the DNS:

  • Primary name server, which contains the master copy of the zone file for the zones it has authority over
  • Secondary name server, which obtains its zone files using a zone transfer from a master name server
  • Master name server, which can provide zone information to secondary name servers
  • Caching-only name server, which does not contain any zone information

Although DNS is used mainly for the Internet, large private TCP/IP internetworks can also use DNS internally with their own name servers. The main advantage of doing this is that host names are friendlier than IP addresses. On smaller TCP/IP networks, hosts files can be used instead of DNS, while on Windows NT–based networks, Windows Internet Naming Service (WINS) is often used for NetBIOS name resolution. Windows NT Server has an installable service called the Microsoft DNS Service, which allows Windows NT servers to function as name servers called DNS servers.

The DNS is also an essential part of Active Directory in Windows 2000. Active Directory uses the DNS to resolve domain names into IP addresses. However, it can also use non-DNS naming conventions to locate objects in the directory. These other naming conventions include

  • The Lightweight Directory Access Protocol (LDAP) naming convention of distinguished names and relative distinguished names (RDNs). This includes LDAP Uniform Resource Locators (URLs).
  • User principal names for identifying users and groups.
  • Security Accounts Manager (SAM) account names for user accounts.
  • Universal Naming Convention (UNC) paths for shared network resources.

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