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
