yf1.a1.b.yahoo.net has no NS records.
NS records for this zone are defined at a1.b.yahoo.net.
NS records for this zone are defined at a1.b.yahoo.net.
Question and response
Question
dig yf1.a1.b.yahoo.net. NS
Answer
The response has no answer records.
Authority
a1.b.yahoo.net. 300 SOA yf1.yahoo.com. hostmaster.yahoo-inc.com. 1773596406 30 30 86400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0