Re: DNS and subdomains

[Mark Andrews <marka () isc org>]

Every domain is a subdomain of something else other than the root. 

access.api.bbc.com is a subdomain of api.bbc.com and a subdomain of
bbc.com and a subdomain of com and a subdomain of . (the root).

All subdomains are domains.  All domains can have subdomains except
those that are maximal size and maximal size - 1. The minimum label
size is 2 (length + value) except for the root which takes 1 octet
(length == 0).

Subdomain is just a relationship to a parent domain.  A domain may or
may not correspond to a zone cut.  All domains have a parent domain
except the root.

Mark

> On 25 Feb 2025, at 12:58, Harry Hoffman via NANOG <nanog () nanog org> wrote:
>
> Hi Folks,
>
> Feel free to tell me this isn't the proper place for my question but given that networking and DNS are hand in hand I 
> thought it might be reasonable to ask here.
>
> In working with several OSINT sources for domain processing it seems like the way domains and subdomains are 
> processed essentially equates subdomains with FQDNs.
>
> For example, several APIs (and even ChatGPT) classify the following:
> access.api.bbc.com
> account-api.api.bbc.com
> account-api.int.api.bbc.com
> account-api.stage.api.bbc.com
> account-api.test.api.bbc.com
> account-cdn.test.api.bbc.com
>
> with subdomains as either:
> all subdomains as api.bbc.com or as subdomains of access.api, account-api.api, account-api.int.api, etc. 
>
> instead of classifying as:
> api.bbc.com
> int.api.bbc.com
> stage.api.bbc.com
> test.api.bbc.com
>
> Has this become common practice? Is there a definitive way to determine subdomains? I seem to recall that "older" dns 
> server software wouldn't allow this but it could be that my memory is faulty.
>
> Thanks!
>
> Cheers,
> Harry

-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka () isc org