Other-level domains

In addition to the top-level, or root, 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.wikipedia.org, wikipedia is the second-level domain.

Next are third-level domains, which are shown 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.

Domains of third or higher levels are also known as subdomains, though this term technically applies to a domain of any level because 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 has been chosen based on the name of a company (e.g., microsoft.com), product or service (e.g., gmail.com). The third level was commonly used to designate a particular host server. Therefore, ftp.wikipedia.org might be an FTP server, www.wikipedia.org would be a World Wide Web server, and mail.wikipedia.org could be an email server. Modern technology 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 serve any real purpose.