Second-level and lower level domains
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.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. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. An example of a working domain with four domain levels is www.sos.state.oh.us. The www preceding the domains is a host name of the World-Wide Web server. Each level is separated by a dot, or period symbol. 'sos' is said to be a sub-domain of 'state.oh.us', and 'state' a sub-domain of 'oh.us', etc. In general, Sub-domains are domains subordinate to their parent domain.
Second-level (or lower-level, depending on the established parent hierarchy) domain names have been created based on the name of a company (e.g., microsoft.com), product or service (e.g., gmail.com). Below these levels, the next domain name component has been 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, each intended to perform only the implied function. Modern technology allows multiple physical servers with either different (cf. load balancing or even identical addresses (cf. anycast) to serve a single hostname or domain name, or multiple domain names to be served by a single computer. The latter is very popular in Web hosting service centers, where service providers host the websites of many organizations on just a few servers.
