We pick m.root-servers.net, one of the 13 root servers (1), and ask it about alti2.aspmx.l.google.com and learn that com is delegated to 13 name servers (2), so we ask m.gtld-servers.net where we learn that google.com is delegated to four name servers, including ns2.google.com (3), so we ask that one and get the following answer: