On Fri, Feb 28, 2025 at 3:18 PM, William Herrin <bill@herrin.us> wrote:

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.

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. It has no DNS records of its own. Only subsidiaries like cat.p.dirtside.com exist and have DNS records.


I'm lost / unclear how you got there… 

RFC1034 says:
"3. DOMAIN NAME SPACE and RESOURCE RECORDS
3.1. Name space specifications and terminology

The domain name space is a tree structure.  Each node and leaf on the
tree corresponds to a resource set (which may be empty).  The domain
system makes no distinctions between the uses of the interior nodes and
leaves, and this memo uses the term "node" to refer to both.
[SNIP]
The domain name of a node is the list of the labels on the path from the
node to the root of the tree. "

and RFC8499 says:
"Domain name:  An ordered list of one or more labels.

      Note that this is a definition independent of the DNS RFCs
      ([RFC1034] and [RFC1035]), and the definition here also applies to
      systems other than the DNS.  [RFC1034] defines the "domain name
      space" using mathematical trees and their nodes in graph theory,
      and that definition has the same practical result as the
      definition here.  Any path of a directed acyclic graph can be
      represented by a domain name consisting of the labels of its
      nodes, ordered by decreasing distance from the root(s) (which is
      the normal convention within the DNS, including this document).  [SNIP]"

Nothing in this says that a "domain name" has to have records of its own, and in fact seem quite clear that this is not the case ("The domain name of a node is the list of the labels on the path from the
node to the root of the tree. " and "Any path of a directed acyclic graph can be represented by a domain name consisting of the labels of its nodes.")

W


"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).

Regards,
Bill Herrin

--
William Herrin
bill@herrin.us
https://bill.herrin.us/