For SEO and usability purposes you may want to redirect your visitors to open your site only through www (http://www.yourdomain.com).
To do this in Magento, you should open the .htaccess file in the folder where your Magento is installed. In it locate the RewriteEngine on line and right after it add the following lines:
Once you do this, save the .htaccess file and log in to the Magento admin area > 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 will start working through http://www.yourdomain.com only!