We pick d.root-servers.net, one of the 13 root servers (1), and ask it about chonso.vinaphone.com.vn and learn that vn is delegated to seven name servers (2), so we ask a.dns-servers.vn where we learn that vinaphone.com.vn is delegated to two name servers, including vdc-hn01.vnn.vn (3), so we ask that one and get the following answer: