Apex domains cannot have CNAME records.
Question and response
Question
dig web.ptd.net. NS
Answer
web.ptd.net. 5 NS dns5.ptd.net.
web.ptd.net. 5 NS dns1.ptd.net.
web.ptd.net. 5 NS dns3.ptd.net.
web.ptd.net. 5 NS dns2.ptd.net.
web.ptd.net. 5 NS dns4.ptd.net.
web.ptd.net. 5 NS dns6.ptd.net.
Authority
The response has no authority records.
Additional
dns1.ptd.net. 5 A 204.186.0.201
dns2.ptd.net. 5 A 207.44.96.129
dns3.ptd.net. 5 A 204.186.0.203
dns4.ptd.net. 5 A 207.44.0.1
dns5.ptd.net. 5 A 204.186.80.169
dns6.ptd.net. 5 A 204.186.80.249
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0