$ dig ns bbm.2ch.net @a.ns.bbm.2ch.net +short
a.ns.bbm.2ch.net.
b.ns.bbm.2ch.net.
$ dig ns bbx.2ch.net @a.ns.bbx.2ch.net +short
dig: couldn't get address for 'a.ns.bbx.2ch.net': not found
$

There is an abnormality in NS?