CNAME vs A record: when to point at an address and when to alias a name

A CNAME takes over its whole name. RFC 1034 says a name with a CNAME can't have any other records, which is why you can't put one on example.com itself: the root of a zone always needs its NS and SOA records. You also can't add a TXT or MX record next to a CNAME on the same name. That catches people out when a service asks for a verification record on www.

Many DNS hosts work around the root problem with a flattened CNAME, also sold as an ALIAS or ANAME record. The host looks up the target itself and answers with plain A and AAAA records, so resolvers never see an alias.

The cost of a CNAME is small. A resolver follows the alias and usually returns the whole chain in one answer. Long chains add a little time on a cold cache and more places for something to break, so keep them short.

The DNS Lookup shows the alias path above each record type, like www.example.com pointing to example.net, so you can see where a CNAME leads and which addresses each resolver ends up at.