We pick h.root-servers.net, one of the 13 root servers (1), and ask it about c-dns.de and learn that de is delegated to six name servers (2), so we ask l.de.net where we learn that c-dns.de is delegated to two name servers, including ns1.c-dns.de (3), so we ask that one and get the following answer:
The two name servers ns1 and ns2.c-dns.de together
There are four domains that use name server group 1.
All of those use only the IPv4 number 83.246.64.172. Worth noting is that four of the domains that only use the IPv4 number 83.246.64.172 use name server group 1.
All of them point only to the IP number 83.246.64.172.
Three of them use the primary name server ns1.c-dns.de. Worth noting is that all of the domains that use the primary name server ns1.c-dns.de use name server group 1.
The IPv4 number 83.246.64.172 only
There are about a hundred domains that only use the IPv4 number 83.246.64.172.