: It's possible exactly the same way that unix.stackexchange.com itself is possible. DNS does not impose any specific hierarchy, like "subdomain – domain – toplevel", you can add as many labels

@Pierce454

It's possible exactly the same way that unix.stackexchange.com itself is possible. DNS does not impose any specific hierarchy, like "subdomain – domain – toplevel", you can add as many labels as you need, and add delegations at any level.

(Take "www.theregister.co.uk" for example. Is "theregister.co.uk" a domain or a subdomain? It's both.)

In other words, the administrators just added "meta.unix.stackexchange.com" to their DNS management software.


For example, in a BIND zone file it might look like this:

...
unix.stackexchange.com. A 198.252.206.140
meta.unix.stackexchange.com. A 198.252.206.140
...


(This lets you see that all levels are functionally equivalent.)
Or like this:

$ORIGIN stackexchange.com.
...
unix A 198.252.206.140
meta.unix A 198.252.206.140
...



So there's a chance that your DNS control panel also allows entering "meta.unix" as a subdomain name. If it doesn't, that's a completely artificial limit decided by your host – DNS itself allows 127 total "levels" (each individual label up to 63 bytes, the complete name up to 253 bytes).

For example, No-IP might have such a limit for marketing/business reasons (to make you buy an actual domain instead of freely creating subdomains off the shared ones.)



For similar reasons, you'll only be able to use bind9 (or nsd, djbdns...) with a purchased domain, not with a No-IP subdomain.


Technically, No-IP could delegate a subdomain to your own bind9 server – it's as simple as adding NS-type records like you would add A/AAAA/CNAME ones:

foobar.no-ip.org. NS ns1.fakeisp.com.
foobar.no-ip.org. NS ns2.fakeisp.com.


(Side note: Yes, NS records only accept names, not IP addresses. For this reason, if you self-host the domain on your own DNS servers, you would need a "glue record":

quux.no-ip.org. NS ns.quux.no-ip.org.
ns.quux.no-ip.org. A 172.16.42.42


But that's a topic for another post.)
But in practice, they won't, for business reasons – they do sell domain names, after all.

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