Apex domains cannot have CNAME records.
Question and response
Question
dig a.subnet.rcn.com. NS
Answer
a.subnet.rcn.com. 5 NS auth1.dns.rcn.net.
a.subnet.rcn.com. 5 NS auth3.dns.rcn.net.
a.subnet.rcn.com. 5 NS auth2.dns.rcn.net.
a.subnet.rcn.com. 5 NS auth4.dns.rcn.net.
Authority
The response has no authority records.
Additional
auth1.dns.rcn.net. 4 A 66.44.1.44
auth1.dns.rcn.net. 4 A 207.172.3.20
auth2.dns.rcn.net. 4 A 207.172.11.14
auth2.dns.rcn.net. 4 A 207.237.14.120
auth3.dns.rcn.net. 4 A 207.96.3.153
auth3.dns.rcn.net. 4 A 207.172.3.21
auth4.dns.rcn.net. 4 A 207.172.3.22
auth4.dns.rcn.net. 4 A 208.59.89.9
auth1.dns.rcn.net. 4 AAAA 2610:80:0:102:0:0:0:44
auth1.dns.rcn.net. 4 AAAA 2610:80:0:10e:0:0:0:20
auth2.dns.rcn.net. 4 AAAA 2610:80:0:100:0:0:0:17
auth2.dns.rcn.net. 4 AAAA 2610:80:0:101:0:0:0:120
auth3.dns.rcn.net. 4 AAAA 2610:80:0:10e:0:0:0:21
auth3.dns.rcn.net. 4 AAAA 2610:80:80:0:0:0:0:153
auth4.dns.rcn.net. 4 AAAA 2610:80:0:103:0:0:0:9
auth4.dns.rcn.net. 4 AAAA 2610:80:0:10e:0:0:0:22
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0