We pick g.root-servers.net, one of the 13 root servers (1), and ask it about dns.ac and learn that ac is delegated to seven name servers (2), so we ask b.ns13.net where we learn that dns.ac is delegated to three name servers, including dns3.dns.ac (3), so we ask that one and get the following answer:
Dns.ac is a domain controlled by three domain name servers at dns.ac themselves. All three of them are on different IP networks. Incoming mail for dns.ac is handled by one mail server also at dns.ac.
Dns.ac uses the primary name server dns1.dns.ac, the three name servers dns1, dns2 and dns3.dns.ac together, hereafter referred to as "name server group 1", and the primary mail server mail.dns.ac.
The three name servers dns1, dns2 and dns3.dns.ac together
There are about sixty domains that use name server group 1.