Apex domains cannot have CNAME records.
Question and response
Question
dig xfinity.com. NS
Answer
xfinity.com. 5 NS dns103.comcast.net.
xfinity.com. 5 NS dns104.comcast.net.
xfinity.com. 5 NS dns105.comcast.net.
xfinity.com. 5 NS dns102.comcast.net.
xfinity.com. 5 NS dns101.comcast.net.
Authority
The response has no authority records.
Additional
dns101.comcast.net. 5 A 69.252.250.103
dns102.comcast.net. 5 A 68.87.85.132
dns103.comcast.net. 5 A 68.87.76.228
dns104.comcast.net. 5 A 68.87.68.244
dns105.comcast.net. 5 A 68.87.72.244
dns101.comcast.net. 5 AAAA 2001:558:fe23:8:69:252:250:103
dns102.comcast.net. 5 AAAA 2001:558:1004:7:68:87:85:132
dns103.comcast.net. 5 AAAA 2001:558:1014:c:68:87:76:228
dns104.comcast.net. 5 AAAA 2001:558:100a:5:68:87:68:244
dns105.comcast.net. 5 AAAA 2001:558:100e:5:68:87:72:244
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0