If you wish to use always http://www.domain.com instead of http://domain.com, in ASP .Net a solution is to modify the file global.asax:

protected void Application_BeginRequest( object sender, EventArgs e){
   //check the request to make it starts with www
   //and is not localhost (dev)
   if (!Request.Url.Host.StartsWith( "www" ) && !Request.Url.IsLoopback)
   {
      //now ... redirect.
       UriBuilder builder = new UriBuilder (Request.Url);
      builder.Host = "www." + Request.Url.Host;
      Response.Redirect(builder.ToString(), true );
   }
}

There are and other solution, like httpd.ini, or depending what you use. In PHP (Apache) I solve this by adding in the top of .htaccess file this code:

Options +FollowSymlinks
RewriteEngine on

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

Why to use this? The 1st  reason is for seo (you will see that in a future article) and another reasons is that it looks more profesional.