We pick a.root-servers.net, one of the 13 root servers (1), and ask it about nsb.nic.uk and learn that uk is delegated to eleven name servers (2), so we ask ns7.nic.uk and get the following answer:
There are eleven domains that use the name server nsb.nic.uk.
All of those use the eleven name servers ns1, ns2, ns3, ns4, ns5, ns6, ns7, nsa, nsb, nsc and nsd.nic.uk together, hereafter referred to as "name server group 1", (example: uk and .uk ).
One of them use the two mail servers mx2 and rav.nic.uk together, hereafter referred to as "mail server group 1".
The eleven name servers ns1, ns2, ns3, ns4, ns5, ns6, ns7, nsa, nsb, nsc and nsd.nic.uk together
One of the eleven domains that use name server group 1 use mail server group 1.
The IP number 156.154.101.3
All of the one host names that point to the IP number 156.154.101.3 point only to that ip number.