sub.xtra.pl has no NS records.
NS records for this zone are defined at xtra.pl.
NS records for this zone are defined at xtra.pl.
Question and response
Question
dig sub.xtra.pl. NS
Answer
The response has no answer records.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0