We pick b.root-servers.net, one of the 13 root servers (1), and ask it about 145.102.in-addr.arpa and learn that in-addr.arpa is delegated to six name servers (2), so we ask a.in-addr-servers.arpa where we learn that 102.in-addr.arpa is delegated to seven name servers, including sec1.authdns.ripe.net (3), so we ask that one and get the following answer: