host21.co154.isl.net.tw has no CNAME records.
Question and response
Question
dig host21.co154.isl.net.tw. NS
Answer
The response has no answer records.
Authority
co154.isl.net.tw. 5562 SOA dns.isl.net.tw. root.isl.net.tw. 2002022202 43200 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0