Redirect from www.domain.com to domain.com

Your website can be accessed with or without the www prefix. Since search engines judges this to be duplicated content, you have to choose either with or without the www prefix. But since some links are outside of your website scope and the search engines might already have indexed your website under both addresses, you can’t change that easily. In this case do a 301 redirect for all http requests that are going to the wrong URL. Example 1- force www prefix
(Add the following code to your .htaccess file)

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

Example 2 – avoid www prefix
(Add the following code to your .htaccess file)

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

As nowadays, there is no a strict technical requirement to use “www” to direct the server, as it was use earlier to direct an application to the right service. Many people (including me) will suggest you not to apply www. and simply stick to “domain.com”.