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Internet Domain Names

In the Domain Name System... there is a hierarchy of names. The root of system is unnamed. There are a set of what are called "top-level domain names" (TLDs). These are the generic TLDs (EDU, COM, NET, ORG, GOV, MIL, and INT), and the two letter country codes from ISO-3166. It is extremely unlikely that any other TLDs will be created.

- Jon Postel; Domain Name System Structure and Delegation ; RFC 1591; March 1994.

Internet domain names come in four main types -- top-level domains, second-level domains, third-level domains, and country domains.

Internet domain names are the alphanumeric identifiers we use to refer to hosts on the Internet, like "". The structure of Internet domain names was first described in RFC 819, and their syntax was first described in RFC 822. You can look up existing Internet domain names with the whois service, and get your own domain name from an accredited registrar.

The following subsections provide information on top-level domains, second-level domains, third-level domains, and country domains.

Top-level domains. Internet domain names are organized by their levels, with the higher levels on the right. For example, for the domain "" the top-level domain is "net", the second-level domain is "", and the third-level domain is "".

Originally, the top-level Internet domains were associated with organizations, mainly ".arpa", ".csnet", ".bitnet", and ".uucp". On January 28, 1986, coincidentally the same day the Challenger space shuttle exploded, those same four organizations met at SRI and agreed to restructure the domain name space into the following eight subject-specific top-level domains, with technical administration performed by ISI:
    Used for computers on the BITNET network.
    Originally stood for "commercial" to indicate a site used for commercial purposes, but it has since become the most well-known top-level Internet domain, and is now used for any kind of site.
    Used by "International" sites, usually NATO sites.
    Used for educational institutions like universities.
    Used for US Government sites.
    Used for US Military sites.
    Originally intended for sites related to the Internet itself, but now used for a wide variety of sites.
    Originally intended for non-commercial "organizations", but now used for a wide variety of sites. Was managed by the Internet Society for awhile.

In December 2002, after a long debate, ICANN approved the following additional top-level Internet domains: .aero, .biz, .coop, .info, .museum, .name, and .pro. In October, 2004, ICANN approved the top-level domains .travel and .post. Periodically new top level domains continue to be approved.

Periodically there are proposals to establish a special top level Internet domain name for compartmentalization of adult material. RFC 3675, in the words of the summary when it was released, "explains why this is an ill considered idea from the legal, philosophical, and particularly, the technical points of view".

Second-level domains. Top-level Internet domains like ".com" are shared by all the organizations in the domain. Second-level domain names like "" and "" are registered by individuals and organizations. Second-level domains are the addresses commonly used to host Internet applications like web hosting and email addressing.

Excluding the top-level domain portion, second-level domain names can have up to 61 characters. For many years, character were restricted to the 26 letters, 10 numbers, or the hyphen character, except the hyphen can't be the first or last character. Under these conditions, there are 36 possibilities for the first and last character of the domain name, and 37 possibilities for the other 59 characters. Therefore, the total number of possible different second level domain names was:

37^59 x 36 x 36

You can see why people say there will always be second-level domain names available. And since the introduction of internationalization of the Internet domain name system (see RFC 3490), the number of possibilities for each character has increased considerably. Thanks to the power of exponentiation, the number of different possible second level domain names is now so large as to be effectively infinite. However, only a few Internet domains correspond to recognizable words, and the vast majority are random looking names like, which not surprisingly was still available when last checked.

Third-level domains. Third-level Internet domain names are created by those that own second-level domains. Third-level domains can be used to set up individual domains for specific purposes, such as a domain for web access and one for mail, or a separate site for a special purpose:


Fourth-level and even higher Internet domains like "" can be also be established. Three of four levels is usually sufficient for most purposes.

Country domains. Each country in the world has its own top-level Internet domain with a unique alphabetic designation. A few countries and example domains are shown below.



Example site


United Arab Emirates


















United States

Organizations in each country are responsible for managing the top-level Internet domain, and then for allocating second-level domains within that domain to people and organizations with interests in that country. The Country Code Domain Name Supporting Organization (CCNSO) is a support organization of ICANN responsible for developing consensus positions and recommending global policies relating to country-code top-level domains. The following resources provide more information about country domains and organizations:

Historical references:


Living Internet content is (c) Copyright William Stewart