_netblock2.seismic.com has no CNAME records.
Question and response
Question
dig _netblock2.seismic.com. NS
Answer
The response has no answer records.
Authority
seismic.com. 789 SOA ns-1254.awsdns-28.org. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0