On Fri, Feb 28, 2025 at 9:11 AM Jay <mysidia@gmail.com> wrote:
Every subdomain is in fact a domain name.
Hi Jay,
Not necessarily.
Yes, necessarily!
Remember my example cat.p.dirtside.com? P.dirtside.com is a subdomain of dirtside.com. It's an administrative grouping of domain names that have a particular characteristic. However, p.dirtside.com is NOT a domain name.
I think you have a different semantic definition to "being a domain name" compared to what the rest of us use.
It has no DNS records of its own. Only subsidiaries like cat.p.dirtside.com exist and have DNS records.
However, that does not make p.dirtinside.com not being a domain name. It would however be what in DNS technical terminology is called "an empty non-terminal" -- a domain name which only exists because it has other domain names "below" it. You would notice that if you used "dig" and queried for the IP address ("A" record type) of p.dirtinside.com, you would get a "NOERROR" status code in the response, but will not get any records in the answer section, because there are none to be returned. This is what is also called a "nodata" response. The same would happen for all other record types. Note that this is different from the response you would get when queriying for a nonexistent domain name, such as q.dirtinside.com (assuming neither it nor any "children" names exist), when you would get a query response status of "NXDOMAIN" ("the queried-for domain name does not exist"). However, that doesn't make the queried-for domain name not be a domain name (or a subdomain for that matter).
"Subdomain" has some funky contradictions to it, some of which can only be resolved with administrative knowledge about the DNS zone they're a part of. That's what makes them a less than useful concept for an outside observer trying to categorize a set of fully qualified domain names (FQDNs).
Again, I suspect you attach different semantics to "subdomain" compared to what most others do. Refer back to Mark Andrews' response. Being a subdomain is not the same as being the name of a zone. Every domain name except the root is a subdomain (of some domain). Being a subdomain is usually referring to another domain, so that one can answer "yes" or "no" to whether "a" is a subdomain of "b", and is answerable purely by the name structure. I would perhaps also point to the latest "DNS Terminology" RFC, ref.: https://datatracker.ietf.org/doc/rfc9499/ As Rubens Kuhl responded, security tools and browsers included use the data from https://publicsuffix.org/ to e.g. determine if HTTP cookies can be shared between different http origins below a given domain name. It tries to delineate the places in the DNS where you find administrative and/or organizational boundaries, which is different from both "being a subdomain (of <a>)" and "being a zone". However, there is nothing *technological* in the DNS which says that you can't start a new public registry and hand out domain names to different organizations under a given DNS domain name (whether it would be successful is another matter). Regards, - Håvard