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 <http://a.b.example.com/>. is a subdomain, but it should never be referred to as a subdomain of EXAMPLE.COM <http://example.com/>. It is only a subdomain of B.EXAMPLE.COM <http://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.