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Domain Name System (DNS) – Part I Name Resolution

By on Mon, 03/10/08 - 11:36pm.

Domain Name System (DNS) is a hierarchical client/server-based distributed database management system that translates Internet domain names such as MICROSOFT.COM to an IP address. It is used because domain names are easier to remember than IP addresses. The DNS clients are called resolvers and the DNS servers are called name servers. The DNS system can be thought of as its own little network. If one DNS server doesn’t know how to translate a particular domain name, it will ask another DNS server. DNS is most commonly associated with the Internet but private networks can also use DNS to resolve computer names and to locate computers within their local networks without being connected to the Internet.

The first TCP/IP networks used HOSTS files to translate from domain names (such as MICROSOFT.COM, ACME.COM or MIT.EDU) to IP addresses. As some networks grew so fast, manually updating and distributing the HOSTS file was not very effective. For the Internet (the largest network that uses DNS), there is not a single organization that is responsible for keeping the DNS system updated. Instead, it is a distributed database that exists on many different name servers around the world, with no one server storing all the information. Because of this, DNS allows for almost unlimited growth.

The most popular implementation of the DNS protocol is the Berkeley Internet Name Domain (BIND), which was developed for the UC Berkeley’s BSD UNIX operating system. The primary specifications for DNS are defined in Requests for Comments (RFC) 1034 and 1035. DNS uses either UDP port 53 or TCP port 53 as the underlying protocol. Windows Server 2003 and Windows Server 2008 include all the necessary software to operate as a DNS server.

The DNS name space describes the hierarchical structure of the DNS database as an inverted logical tree structure. Each node on the tree is a partition of the name space called a domain. Domains can be further partitioned at node points within the domain into subdomains. The names of the domain and subdomains can be up to 63 characters long.

At the top of the tree is known as the root domain. It is sometimes shown as a period (.) or as empty quotation marks (“”), indicating a null value. Immediately below the root domain, you will find the top-level domains. The top-level domains indicate a country, region or type of organization. Three letter codes indicate the type of organization. For example, COM indicates Commercial (business) and EDU stands for educational institution.

The traditional top level domains include:

  • com - Commercial
  • edu - Educational
  • gov - Government
  • int - International Organization
  • mil - Military
  • net - Network related
  • org - Miscellaneous Organization

Two letter codes indicate countries, which follow the International Standard 3166. For example, CA stands for Canada, AU for Australia, FR for France and UK for United Kingdom. For a list of two letter codes, go to the website. 

The second-level domain names are variable-length names registered to an individual or organization for use on the Internet. These names are almost always based on the appropriate top-level domain, depending on the type of organization or geographic location where a name is used.  


  • MICROSOFT.COM - Microsoft Corporation
  • CISCO.COM - Cisco Corporation
  • MTI.EDU - MTI University
  • ED.GOV - United States Department of Education
  • ARMY.MIL - United States Army
  • W3.ORG - World Wide Web Consortium
  • NATO.INT - North Atlantic Treaty Organization
  • PM.GOV.AU - Prime Minister of Australia 

The second-level domain names must be registered by the authorized party. For example, for years, Network Solutions Inc. ran a government-sanctioned monopoly on registrations for .COM, .NET and .ORG domain names. But as the US government handed the control of the Internet to an international body, several companies now handle the registration of these three letter codes. Note: Since the most of the common top-level domains names are already taken, some countries such as Tonga (TO) and Tuvalu (TV) are selling their domain name. Therefore, some commercial and user sites may be using one of these two letter codes. This is especially true with the TV domain name since it is easily linked to television.

Subdomain names are additional names that an organization can create that are derived from the registered second-level domain name. The subdomain allows an organization to divide a domain into a department or geographical location, allowing the partitions of the domain name space to be more manageable. A subdomain must have a contiguous domain name space. This means that the domain name of a zone (child domain) is the name of that zone added to the name of the domain or parent domain.

A host name is a name assigned to a specific computer within a domain or subdomain by an administrator to identify the TCP/IP host. Multiple host names can be associated with the same IP address, although only one host name can be assigned to a computer. If the DNS is seen as a tree, it represents the leaf or object of the tree. Much like a subdomain, it is the leftmost label of the DNS domain name. The host name can then be used in place of an IP address such as the PING or other TCP/IP utilities. Total length of an FQDN cannot exceed 255 characters. Note: The host name does not have to be the same as the NetBIOS (computer) name.

A fully qualified domain name (FQDN) describes the exact position of a host (computer) within the domain hierarchy and it is considered to be complete. When used in a DNS domain name, it is stated by a trailing period (.) to designate the name of the host is located off the root or highest level of the domain hierarchy.



  • COM indicating a commercial business
  • ACME is the name of the domain name
  • SALES is the name of the subdomain
  • SERVER1 is the name of the server located within the SALES subdomain

Part II will discuss the Domain Zones and Resouce Records that make up the DNS system and use for name resolution.