About [HttpPost] action filter

See more:

MVC4

When I place [HttpPost] filter on Index action of my controller. I get error The resource cannot be found.
Please any body explain me why is this happen.

public class BooksController : Controller
   {
       public ActionResult Index()
       {
           return View();
       }

above code run easily when i write localhost:1530/Book in browser.
But when I write [HttpPost] on it (as below code) it shows error.

public class BooksController : Controller
    {
        [HttpPost]
        public ActionResult Index()
        {
            return View();
        }

my routeconfig code is below

public class RouteConfig
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                name: "Default",
                url: "{controller}/{action}/{id}",
                defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
            );
        }
    }

Explain me why is this happen.