We pick l.root-servers.net, one of the 13 root servers (1), and ask it about dns.at and learn that at is delegated to eight name servers (2), so we ask u.ns.at where we learn that dns.at is delegated to two name servers, including ns1.kasec.at (3), so we ask that one and get the following answer:
Dns.at is a domain controlled by two domain name servers at kasec.at. Both are on different IP networks. Incoming mail for dns.at is handled by two mail servers at dns.at themselves. Both are on different IP networks.