A top-level domain is the last label in a domain name, the part to the right of the final dot. In example.com the top-level domain is com; in bbc.co.uk it is uk. Because DNS is organized as a tree, the top-level domains sit directly beneath the root and form the first branching point of the entire namespace. Everything else — second-level names, subdomains, individual hosts — hangs below one of these.
The original set was laid down in RFC 920, “Domain Requirements,” written by Jon Postel and Joyce Reynolds in October 1984. It defined a temporary ARPA domain for “the current ARPA-Internet hosts” and a set of category domains: GOV for government, EDU for education, COM for commercial entities, MIL for military, and ORG for “any other domains meeting the second level requirements.” Crucially, the same document also provided for “country codes using ISO Standard alpha-2 identifiers,” establishing the two families of TLDs that still organize the namespace: generic and country-code.
A decade later, RFC 1591, “Domain Name System Structure and Delegation” (Jon Postel, March 1994), restated and refined the structure for a much larger internet. It described “a set of what are called ‘top-level domain names’ (TLDs),” listing “the generic TLDs (EDU, COM, NET, ORG, GOV, MIL, and INT), and the two letter country codes from ISO-3166.” It clarified intended uses — COM “for commercial entities, that is companies,” GOV “for any kind of government office or agency” — and set out that “the country code domains (for example, FR, NL, KR, US) are each organized by an administrator for that country.”
These RFCs established a pattern that has held even as the system expanded enormously. The generic/country-code split, the principle of delegation, and the role of a central coordinator all date from this early work. The set of TLDs is no longer fixed — hundreds of new generic TLDs have since been added — but they all still occupy the same position at the top of the DNS tree, the first decision a resolver makes when it walks a name from the root downward.