d.edu-servers.net has no NS records.
NS records for this zone are defined at edu-servers.net.
NS records for this zone are defined at edu-servers.net.
Question and response
Question
dig d.edu-servers.net. NS
Answer
The response has no answer records.
Authority
edu-servers.net. 86400 SOA av4.nstld.com. nstld.verisign-grs.com. 1772410707 3600 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0