So what’s the reason behind this?

WWW [short form of World Wide Web] is not really a useful thing on internet. It work like a subdomain, where all your documents, HTML and other files reside which will be publicly viewable. This is what Wikipedia says about it..

The letters “www” are commonly found at the beginning of Web addresses because of the long-standing practice of naming Internet hosts (servers) according to the services they provide. So for example, the host name for a Web server is often “www”; for an FTP server, “ftp”; and for a USENET news server, “news” or “nntp” (after the news protocol NNTP). These host names appear as DNS subdomain names, as in “www.example.com”.

As a matter of fact, use of the www subdomain is redundant and time consuming to communicate. The internet, media, and society are all better off without it.

So the current web administrators are practicing the no www thing. But as people are used to use the www, so they keep the both settings in their server. Some of the admins redirect all the traffic to www.example-domain.com like Google do when some of them redirect every thing to example-domain.com. I will go with the later one, it should be the standard. Even Google crawlers like the no-www address.

So if you want to make your default address example-domain.com and want to redirect www.example-domain.com to example-domain.com, so that you can catch everyone’s traffic when they hit on your site, just open .htaccess file in your root directory [assuming you are using apache httpd] and write following lines.

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

There is an organisation named No-www.org working on making internet www free Check them.

To validate your domain if it supports no-www, please use this link : http://no-www.org/verify.php.