We pick a.root-servers.net, one of the 13 root servers (1), and ask it about pornorama.com.pl and learn that pl is delegated to eight name servers (2), so we ask e-dns.pl where we learn that pornorama.com.pl is delegated to two name servers, including ns2.aftermarket.pl (3), so we ask that one and get the following answer:
One percent of them use the two name servers ns1 and ns2.dropped.pl together, hereafter referred to as "name server group 1", (example: legendarny.net and hostelik.pl ).
At least one of them use the two name servers ns1 and ns2.aftermarket.pl together, hereafter referred to as "name server group 2", (example: wonderbox.com.pl ).