Domain Name Service (DNS)

Domain Name System (or Service) is an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to 198.105.232.4. To the user and application process this translation is a service provided either by the local host or from a remote host via the Internet. The DNS server (or resolver) may communicate with other Internet DNS servers if it cannot translate the address itself. The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.


DNS Name Structure

DNS names are constructed hierarchichally. The highest level of the hierarchy being the last component or label of the DNS address. Labels can be up to 63 characters long and are case insensitive. A maximum length of 255 characters is allowed. Labels must start with a letter and can only consist of letters, digits and hyphens. [Unfortunately some administrators construct names that start with digits. This is wrong and can easily cause problems with software that simply inspects the first character of a host address to determine whether a DNS name or an IP address has been quoted.]

DNS addresses can be relative or fully qualified. A fully qualified address includes all the labels and is globally unique. A relative address can be converted by appending the local domain information.


3-Letter Codes

The DNS was orginally introduced in the United States of America and the final component of an address was intended to indicate the type of organisation hosting the computer. Some of the three letter final labels (edu, gov, mil) are still only used by organisations based in the USA, others can be used anywhere in the world. The three letter codes are:

CodeMeaning
.comCommercial
.orgNon-profit Organization
.netInternet Service Provider
.eduEducational Institution
.govGovernment
.milMilitary
.intInternational Organization



Last Updated: 17 September 1999, 02:27