Overview
The Domain Name System (DNS) has a tree structure or hierarchy, with each node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain. For example, mail.example.com and calendar.example.com are subdomains of the example.com domain, which in turn is a subdomain of the com top-level domain (TLD). A "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each DNS label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
Read more about this topic: Subdomain