We pick f.root-servers.net, one of the 13 root servers (1), and ask it about 18.225.91.81.in-addr.arpa and learn that in-addr.arpa is delegated to six name servers (2), so we ask f.in-addr-servers.arpa where we learn that 81.in-addr.arpa is delegated to six name servers, including tinnie.arin.net (3), so we ask that one and get the following answer: