Apex domains cannot have CNAME records.
Question and response
Question
dig ikb-net.at. NS
Answer
ikb-net.at. 5 NS ispconfig3.ikbnet.at.
ikb-net.at. 5 NS ns5.ikbnet.at.
ikb-net.at. 5 NS ns4.ikbnet.at.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0