On 23/03/18 01:17, Greg Wooledge wrote: > 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. >
Thanks - Having read that paragraph of the RFC, it doesn't seem to require any particular number of levels, only that all that exist are present. Richard
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