CNAME records are used to identify multiple domain names that point to the same IP address.
An A record points directly to an IP address.
Essentially a CNAME record is used when you don’t have access to an A record, for example when you have a WordPress blog on a sub-domain (wordpress.site.com)
CNAME Vs. 301 Redirects
The main difference between the two methods is that a 301 redirect can target a Web page, but a DNS canonical record specializes in renaming the entire site. Each technique is appropriate to different circumstances.
SEO Clarification
Choose a default canonical path –
Every website needs a ‘canonical’ or definitive web address of the site.
Generally this is the non-www (bare domain name without the “www” in front).
Search engines view both the “www.site.com” and “site.com” as two different addresses so its important to point the www version to the canonical name (or the non-www to the www).
How does this help SEO?
Search engines can then merge all results from these two addresses, consolidating visitor counts and removing the danger of being demoted or suspended for having two sites with the same content.
Which is the definitive address of a site?
The definitive address of a site is registered as an “A” record.
Why use a CNAME record?
Canonical records (indicated by the “CNAME” record) point other addresses to the canonical address, which is the “A” record (above).
DNS vocabulary terms
DNS
The Domain Name System (DNS) translates human readable domain names to numerical IP addresses
DNS hosting service
Runs DNS servers; often included with domain name registration
Domain name registrar
Manages reservation of Internet domain names
CNAME record
Canonical Name record points to a domain name, which then maps to IP addresses; used for high availability
A record
Maps a domain name to an IPv4 address
AAAA record
Maps a domain name to an IPv6 address
IPv4
Core Internet protocol for routing traffic
IPv6
Core routing Internet protocol intended to replace IPv4, supports more addresses
TTL
Length of time for requests to a DNS server to be cached; measured in seconds
