Apex domains cannot have CNAME records.
Question and response
Question
dig isdsl.net. NS
Answer
isdsl.net. 4 NS demeter.is.co.za.
isdsl.net. 4 NS jupiter.is.co.za.
isdsl.net. 4 NS titan.is.co.za.
Authority
The response has no authority records.
Additional
titan.is.co.za. 5 A 196.33.171.36
demeter.is.co.za. 5 A 196.26.5.8
jupiter.is.co.za. 5 A 196.4.160.3
jupiter.is.co.za. 5 AAAA 2c0f:fc00:53:4160:0:53:a:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0