For SEO and usability purposes you may want to redirect your visitors to open your site only through http://www.yourdomain.com.

Some applications, including Magento, however, require additional modifications to make this redirection work properly.

First, you should open the .htaccess file in the Magento folder. In it locate the "RewriteEngine on" line and right after it add the following lines:

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

Once you do this, save the .htaccess file and log in to the administrative end of Magento.

Then go to the System > Configuration menu and from the left panel click the "Web" button.

Unfold the "Unsecured" set of options and change the "Base URL" option from http://yourdomain.com to http://www.yourdomain.com.

Save the changes and your Magento website will be running with www.yourdomain.com only!