We pick l.root-servers.net, one of the 13 root servers (1), and ask it about backup-dc.tuoitre.net.vn and learn that vn is delegated to seven name servers (2), so we ask f.dns-servers.vn where we learn that tuoitre.net.vn is delegated to two name servers, including mail.tuoitre.net.vn (3), so we ask that one and get the following answer: