Heya, Shumon! Great to hear from you and thanks for adjusting my understanding. It's also a good reminder to go read the RFCs so that I can eliminate assumptions :-)
Cheers, Harry On Fri, Feb 28, 2025 at 12:29 PM Shumon Huque <shu...@gmail.com> wrote: > 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. > >
