mailb.fpc.ru has no NS records.
NS records for this zone are defined at fpc.ru.
NS records for this zone are defined at fpc.ru.
Question and response
Question
dig mailb.fpc.ru. NS
Answer
The response has no answer records.
Authority
fpc.ru. 600 SOA ns3-l2.nic.ru. ast.resourceservice.ru. 65012932 14400 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0