On Fri, Feb 28, 2025 at 12:18 PM Harry Hoffman via NANOG <nanog@nanog.org> wrote:

This is exactly the logic that I was operating under: A.B.EXAMPLE.COM.  is a subdomain, but it should never be referred to
as a subdomain of EXAMPLE.COM.   It is only a subdomain of B.EXAMPLE.COM.

Hey Harry, long time no see :)

No, actually your statement is not correct. All of those are subdomains example.com.

To quote the DNS Terminology RFC ( https://datatracker.ietf.org/doc/html/rfc9499 ):

   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".

 

Shumon.