Also refered to as a 3rd level domain is very simply, a domain that is part of a main domain. “” is a subdomain. “.com” is the top level domain, “directom” is the second level, and “www” is the third. Subdomains can be created at any time with no limit and with out a registrar. A common reason to create subdomains would be to differentiate a sector of your business such as “” or “”