f4.nstld.com has no NS records.
NS records for this zone are defined at nstld.com.
NS records for this zone are defined at nstld.com.
Question and response
Question
dig f4.nstld.com. NS
Answer
The response has no answer records.
Authority
nstld.com. 86400 SOA av4.nstld.com. nstld.verisign-grs.com. 1771892308 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0