In the Domain Name System (DNS) hierarchy, a subdomain is a domain that's part of a larger domain. 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's part of a larger domain, the only domain that isn't 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 do… (More on Subdomain)