We pick d.root-servers.net, one of the 13 root servers (1), and ask it about tech.vn and learn that vn is delegated to seven name servers (2), so we ask f.dns-servers.vn where we learn that tech.vn is delegated to two name servers, including ns1.domainvietnam.com (3), so we ask that one and get the following answer:
Tech.vn is a domain controlled by two domain name servers at domainvietnam.com having a total of two IP numbers. Two domain name servers have the same IP number. Both are on the same IP network. Incoming mail for tech.vn is handled by one mail server at kcvn.com having a total of 14 IP numbers. Some of them are on the same IP network. Tech.vn has one IP number (220.127.116.11), but the reverse is win11.kcc.vn.