Also referred to as a 3rd-level domain, a subdomain 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 without a registrar, though you may want to consider the SEO ramifications. A common reason to create subdomains would be to differentiate a sector of your business such as “” or “”