DNS/TLDについて、ここに記述してください。
https://datatracker.ietf.org/doc/html/rfc8499
Domain name: An ordered list of one or more labels.
- Note that this is a definition independent of the DNS RFCs ([RFC1034] and [RFC1035]), and the definition here also applies to systems other than the DNS.
- [RFC1034] defines the "domain name space" using mathematical trees and their nodes in graph theory, and that definition has the same practical result as the definition here.
- Any path of a directed acyclic graph can be represented by a domain name consisting of the labels of its nodes, ordered by decreasing distance from the root(s) (which is the normal convention within the DNS, including this document). A domain name whose last label identifies a root of the graph is fully qualified; other domain names whose labels form a strict prefix of a fully-qualified domain name are relative to its first omitted node. Also note that different IETF and non-IETF documents have used the term "domain name" in many different ways. It is common for earlier documents to use "domain name" to mean "names that match the syntax in [RFC1035]", but possibly with additional rules such as "and are, or will be, resolvable in the global DNS" or "but only using the presentation format".
Top-Level Domain (TLD): A Top-Level Domain is a zone that is one
- layer below the root, such as "com" or "jp". There is nothing special, from the point of view of the DNS, about TLDs. Most of them are also delegation-centric zones (defined in Section 7), and there are significant policy issues around their operation. TLDs are often divided into sub-groups such as Country Code Top-Level Domains (ccTLDs), Generic Top-Level Domains (gTLDs), and others; the division is a matter of policy and beyond the scope of this document.
Subdomain: "A domain is a subdomain of another domain if it is
- contained within that domain. This relationship can be tested by seeing if the subdomain's name ends with the containing domain's name." (Quoted from [RFC1034], Section 3.1) For example, in the host name "nnn.mmm.example.com", both "mmm.example.com" and "nnn.mmm.example.com" are subdomains of "example.com". Note that the comparisons here are done on whole labels; that is, "ooo.example.com" is not a subdomain of "oo.example.com".