We pick m.root-servers.net, one of the 13 root servers (1), and ask it about www18.nhac.vui.vn and learn that vn is delegated to seven name servers (2), so we ask vn.cctld.authdns.ripe.net where we learn that vui.vn is delegated to two name servers, including slv8.24h.com.vn (3), so we ask that one and get the following answer: