231.196.220.91.in-addr.arpa has no NS records.
NS records for this zone are defined at 196.220.91.in-addr.arpa.
NS records for this zone are defined at 196.220.91.in-addr.arpa.
Question and response
Question
dig 231.196.220.91.in-addr.arpa. NS
Answer
The response has no answer records.
Authority
196.220.91.in-addr.arpa. 86400 SOA dns10.copyleft.no. hostmaster.copyleft.no. 3000010701 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0