_sips._tcp.ces.net has no NS records.
NS records for this zone are defined at ces.net.
NS records for this zone are defined at ces.net.
Question and response
Question
dig _sips._tcp.ces.net. NS
Answer
The response has no answer records.
Authority
ces.net. 900 SOA nsa.cesnet.cz. hostmaster.cesnet.cz. 2019072261 28800 7200 1209600 900
Additional
. 0 OPT ; payload 1472, xrcode 0, version 0, flags 0