_netblocks.viacom.com has no NS records.
NS records for this zone are defined at viacom.com.
NS records for this zone are defined at viacom.com.
Question and response
Question
dig _netblocks.viacom.com. NS
Answer
The response has no answer records.
Authority
viacom.com. 3600 SOA dns1.p09.nsone.net. hostmaster.nsone.net. 1655182652 600 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0