On Thu, Mar 22, 2018 at 12:04:07AM -0000, Dan Purgert wrote: > Richard Hector wrote: > > I often see this alluded to, but struggle to find evidence - why > > shouldn't there be a postmaster@com, for example? Or perhaps cic@mil? > > It is my understanding that the TLDs are not themselves valid domains. > That is, a valid domain is by definition "domain.tld". > > I could be entirely wrong though.
>From <https://en.wikipedia.org/wiki/Fully_qualified_domain_name>: A fully qualified domain name (FQDN), sometimes also referred to as an absolute domain name,[1] is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including at least a second-level domain and a top-level domain.[2] Footnote [2]: April N. Marine; Joyce K. Reynolds; Gary Scott Malkin (March 1994). "Questions About the Domain Name System". Answers to Commonly asked "New Internet User" Questions. IETF. sec. 5. doi:10.17487/RFC1594. RFC 1594. Retrieved 29 April 2013. "If you think of the DNS as a tree-structure with each node having its own label, a fully qualified domain name for a specific node would be its label followed by the labels of all the other nodes between it and the root of the tree." RFC 1594 <https://tools.ietf.org/html/rfc1594>: A Fully Qualified Domain Name (FQDN) is a domain name that includes all higher level domains relevant to the entity named. If you think of the DNS as a tree-structure with each node having its own label, a Fully Qualified Domain Name for a specific node would be its label followed by the labels of all the other nodes between it and the root of the tree. For example, for a host, a FQDN would include the string that identifies the particular host, plus all domains of which the host is a part up to and including the top-level domain (the root domain is always null). Now, that's from 1994, and it's an "Informational" RFC ("does not specify an Internet standard of any kind"), but that's probably as authoritative as you'll get.
