Though it’s less popular these days, some people desire to have multiple domain names pointing to the same site. For example, say you have three domain names:

www.example.com
www.example.net
www.example.org

The problem is that, especially if you market all three domains, people are free to link to any of these domains. That’s a major duplicate content issue. You must pick a “standard” domain and permanently redirect the other domains to that domain.
Let’s pick www.example.com. This is how to do it with mod_rewrite:

RewriteEngine on
RewriteCond % {HTTP_HOST} !^www.example.com
RewriteRule ^(.*)$ http://wwww.example.com/$1 [R=301,L]

Done! Now everything will get redirected to www.example.com. Let’s analyze the rule in detail.

This is the first time you’re using RewriteCond to place a condition for the rule that follows. In this case, you’re interested in verifying that the site has been accessed through www.example.com. Take another look at the RewriteCond line:

RewriteCond % {HTTP_HOST} !^www.example.com

This line specifies a condition that is true when the host name (HTTP_HOST) is not(!)www.example.com. The rewrite rule captures the entire query string of the original URL, as (.*), and passes it to http://www.example.com, doing a 301 redirect to the new location. This way, for example, a query to http://www.example/org?query=string would be 301 redirected to http://www.example.com?query=string.

read more on http://www.parasbabbar.in