We pick c.root-servers.net, one of the 13 root servers (1), and ask it about j.mu and learn that mu is delegated to four name servers (2), so we ask fork.sth.dnsnode.net where we learn that j.mu is delegated to two name servers, including ns2.mailart.com (3), so we ask that one and get the following answer:
J.mu is a domain controlled by two domain name servers at mailart.com. Both are on the same IP network. Incoming mail for j.mu is handled by one mail server at j.mu themselves. J.mu has one IP number (18.104.22.168), but the reverse is ?.
All of the several hundred of domains that use the primary name server ns2.mailart.com use name server group 1 (example: amigo.cx, 007.ac and ma.tc ). Worth noting is that all of the domains that use name server group 1 use the primary name server ns2.mailart.com.
The two name servers ns2 and ns3.mailart.com together
There are several hundred of domains that use name server group 1.