TTNET-SERVERS.core.ttnet.cz has no CNAME records.
Question and response
Question
dig TTNET-SERVERS.core.ttnet.cz. NS
Answer
The response has no answer records.
Authority
core.ttnet.cz. 1156 SOA ns.ttnet.cz. hostmaster.ttnet.cz. 2002352068 28800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0