Canonical URL's explained - www. Vs non www.

Google sees subdomains as separate domain names and the www. prefix is just another subdomain. Therefore, it’s important to remove any possible confusion on their part by eliminating one of the two versions from displaying at all. The Big SEO's own preference is to always use the www.

Inside Google Webmaster tools you can go to your domain account then go to Tools and to “set preferred domain” which will let them know that you would prefer to use the www.

This step is not enough on its own however, (some people are even skeptical that this works at all) and it’s best to take care of it at the source, on your server, by simply not allowing the duplicate version to show in the first place.

On an Apache server using mod rewrite, we have have placed the following code in MY .htaccess file to eliminate the appearance of the non www version of any page. You can do the same, (but using your OWN DOMAIN NAME)

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} ^pdxtc.com [NC]
RewriteRule ^(.*)$ http://www.pdxtc.com/$1 [L,R=301]

Hope this helps!