Apex domains cannot have CNAME records.
Question and response
Question
dig inaddr.ip-pool.com. NS
Answer
inaddr.ip-pool.com. 5 NS ns1.nameserverservice.de.
inaddr.ip-pool.com. 5 NS ns2.nameserverservice.de.
Authority
The response has no authority records.
Additional
ns1.nameserverservice.de. 5 A 85.25.128.21
ns2.nameserverservice.de. 5 A 85.25.255.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0