Every domain name ends in a top-level domain (TLD) name, which is always either one of a small list of generic names (three or more characters), or a two characters territory code based on ISO-3166 (there are few exceptions and new codes are integrated case by case). Top-level domains are sometimes also called first-level domains.
Generic top-level domain
The generic top-level domain (gTLD) extensions are:
A generic top-level domain (gTLD) is a top-level domain used (at least in theory) by a particular class of organization. These are three or more letters long, and are named for the type of organization that they represent (for example, .com for commercial organizations). The following gTLDs currently exist (as does .arpa, which is sometimes considered a gTLD):
- .aero – for the air transport industry
- .biz – for business use
- .cat – for Catalan language/culture
- .com – for commercial organizations, but unrestricted
- .coop – for cooperatives
- .edu – for post-secondary educational establishments
- .gov – for governments and their agencies in the United States
- .info – for informational sites, but unrestricted
- .int – for international organizations established by treaty
- .jobs – for employment-related sites
- .mil – for the US military
- .mobi – for sites catering to mobile devices
- .museum – for museums
- .name – for families and individuals
- .net – originally for network infrastructures, now unrestricted
- .org – originally for organizations not clearly falling within the other gTLDs, now unrestricted
- .pro – for certain professions
- .tel – for services involving connections between the telephone network and the Internet (added March 2, 2007)
- .travel – for travel agents, airlines, hoteliers, tourism bureaus, etc.
The following gTLDs are in the process of being approved, and may be added to the root nameservers in the near future:
- .asia – for the Asian community
- .post – for postal services
- .geo – for geographically related sites
- .cym – for Welsh language/culture
In addition to the top-level domains, there are second-level domain (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain en.ozdomainer.com, “ozdomainer” is the second-level domain.
On the next level are third-level domains. These domains are immediately to the left of a second-level domain. In the en.wikipedia.org example, “en” is a third-level domain. There can be fourth and fifth level domains and so on, with virtually no limitation. An example of a working domain with five levels is www.sos.state.oh.us. Each level is separated by a dot or period symbol between them.
Domains of third or higher level are also known as subdomains, though this term technically applies to a domain of any level, since even a top-level domain is a “subdomain” of the “root” domain (a “zeroth-level” domain that is designated by a dot alone).
Traditionally, the second level domain was the name of the company or the name used on the internet. The third level was commonly used to designate a particular host server. Therefore, ftp.ozdomainer.com might be an FTP server, www.ozdomainer.com would be a World Wide Web Server, and mail.ozdomainer.com could be an email server. Modern technology now allows multiple servers to serve a single subdomain, or multiple protocols or domains to be served by a single computer. Therefore, subdomains may or may not have any real purpose.