public static void RegisterRoutes(RouteCollection routes)
{

   routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
   routes.MapRoute("Default2", "", new { controller = "Home", action = "NullUrl", id = UrlParameter.Optional });
   routes.MapRoute( "Default", "{id}", new { controller = "Home", action = "Index", id = UrlParameter.Optional });

}

  public class HomeController : Controller
    {
        //
        // GET: /Home/

        public ActionResult Index(string id)
        {
            return View();
        }

        public ActionResult NullUrl()
        {
            return View();
        }
    }