Apex domains cannot have CNAME records.
Question and response
Question
dig fr.colt.net. NS
Answer
fr.colt.net. 5 NS ns1.coltfrance.com.
fr.colt.net. 5 NS ns0.coltfrance.com.
Authority
The response has no authority records.
Additional
ns0.coltfrance.com. 5 A 84.14.241.56
ns1.coltfrance.com. 5 A 212.74.78.59
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0