Apex domains cannot have CNAME records.
Question and response
Question
dig sdn24x7.com. NS
Answer
sdn24x7.com. 4 NS ns1.sdncommunications.com.
sdn24x7.com. 4 NS ns5.sdncommunications.com.
sdn24x7.com. 4 NS ns2.sdncommunications.com.
Authority
The response has no authority records.
Additional
ns1.sdncommunications.com. 4 A 66.231.7.62
ns2.sdncommunications.com. 4 A 66.231.0.62
ns5.sdncommunications.com. 4 A 45.33.28.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0