In the Domain Name System (DNS) hierarchy, 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
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