Apex domains cannot have CNAME records.
Question and response
Question
dig net240.192.115.61.in-addr.arpa. NS
Answer
net240.192.115.61.in-addr.arpa. 5 NS ns1.internetfacilities.ne.jp.
net240.192.115.61.in-addr.arpa. 5 NS ns2.iprevolution.co.jp.
Authority
The response has no authority records.
Additional
ns1.internetfacilities.ne.jp. 5 A 61.115.192.244
ns2.iprevolution.co.jp. 5 A 61.115.192.18
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0