What is a sub-level domain?
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 isn't
also a subdomain is the root domain.[2] 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.