We pick c.root-servers.net, one of the 13 root servers (1), and ask it about ori.vn and learn that vn is delegated to seven name servers (2), so we ask b.dns-servers.vn and get the following answer:
Seven of the ten domains that use the primary name server ns1.dlcvn.com use the two name servers ns1 and ns2.dlcvn.com together (but neither ns1 nor ns2.dlcvietnam.com ), hereafter referred to as "name server group 1". Worth noting is that all of the domains that use name server group 1 use the primary name server ns1.dlcvn.com.
The IPv4 number 202.78.227.17 only
There are about seventy domains that only use the IPv4 number 202.78.227.17.
The two name servers ns1 and ns2.dlcvn.com together
There are seven domains that use name server group 1.
All of those use the primary name server ns1.dlcvn.com. Worth noting is that seven of the domains that use the primary name server ns1.dlcvn.com use name server group 1.
Three of them use only the IPv4 number 202.78.227.17. Worth noting is that three of the domains that only use the IPv4 number 202.78.227.17 use name server group 1.