Re: DNS and subdomains

[Harry Hoffman via NANOG <nanog () nanog org>]

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 <shuque () 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.