Apex domains cannot have CNAME records.
Question and response
Question
dig ip-exchange.de. NS
Answer
ip-exchange.de. 5 NS ns.ext-dc.de.
ip-exchange.de. 5 NS ns.ext-dc.net.
ip-exchange.de. 5 NS ns.ext-dc.com.
Authority
The response has no authority records.
Additional
ns.ext-dc.de. 5 A 62.146.108.5
ns.ext-dc.com. 5 A 212.123.108.5
ns.ext-dc.net. 5 A 185.234.108.5
ns.ext-dc.de. 5 AAAA 2a01:138:101:15:0:0:0:5
ns.ext-dc.com. 5 AAAA 2a01:138:101:16:0:0:0:5
ns.ext-dc.net. 5 AAAA 2a01:138:101:14:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0