This is hypothetical. Been learning about DNS and I'm curios. Say I have a web server and an authoritative dns server and I want to host other sites. Do I need to purchase a domain name for my web server? Or is it not needed. I can just insert dns records (NS and A of my dns) of the domains I'm hosting to point to my authoritative dns server when that domains are requested. Then I can have CNAME records on my dns server that points all those sites to my web server. Of course I have one A record that maps the name of my web server to its IP address.
Considering things in the strictest way possible, you really only need a domain for your name server (so you can hand out e.g. ns1.yourdomain.com, ns2.yourdomain.com). A records for clients can then be created to point to your web server e.g.
clientdomain.com. IN A 184.108.40.206
Where 220.127.116.11 is your web server IP (without a domain).
However, as soon as you introduce CNAME records (regardless of who is hosting the DNS services), you need a domain for the web server:
yourdomain.com. IN A 18.104.22.168 sub IN A 22.214.171.124 client IN CNAME sub.yourdomain.com
A records map to IP addresses, CNAME (Canonical Name) records map to domains.