We pick a.root-servers.net, one of the 13 root servers (1), and ask it about mbl-109-32-20.dsl.net.pk and learn that pk is delegated to three name servers (2), so we ask root-s.pknic.pk where we learn that dsl.net.pk is delegated to three name servers, including ns1.dsl.net.pk (3), so we ask that one and get the following answer: