Re: DNS and subdomains

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

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/>.

On Fri, Feb 28, 2025 at 12:11 PM Jay <mysidia () gmail com> wrote:

> On Fri, Feb 28, 2025 at 10:05 AM Stephane Bortzmeyer via NANOG
> <nanog () nanog org> wrote:
> > On Tue, Feb 25, 2025 at 07:36:16AM -0500,
>
> Yes.  I believe the confusion is that some documentation,
> APIs, or software incorrectly obfuscate the concept of a domain
> and take that the domain is only the part registered with a domain
> registrar.
>
> E.g. "EXAMPLE.COM" would be a domain, and in "B.C.EXAMPLE.COM"
> the C.EXAMPLE.COM would be disregarded and considered part of Example.com
> leading to the strange claims that you have a "B.C" subdomain of
> example.com
> or that B.c.example.com is a subdomain of example.com
> But that would be completely non-standard.
>
> Every subdomain is in fact a domain name.   And every domain under the root
> is a subdomain of the label directly above it and ONLY the one
> specific DNS label
> directly above that particular subdomain.
>
> And the management or registration structure of the DNS; the zone
> boundaries users
> refer to, have nothing to do with which names are actually a domain or
> subdomain.
>
> A subdomain is exclusively the direct descendent of another domain
> which is observed solely by the addition of one dot and label on the
> lefthand side.
>
> Take for example:     A.B.EXAMPLE.COM.   Assume that FQDN exists.
>
> Given that exists you are guaranteed that A.B.EXAMPLE.COM. is a domain,
> B.EXAMPLE.COM. is a domain,  EXAMPLE.COM. is a domain, COM.
> is a domain,  and .  is a domain.
>
> 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.
>
>
> --
> -J