Web Forms :: How To Handle 404 Errors When Using URL Routing

I have been using using System.Web.Routing for url routing in global.asax file  code is as below.

public static void RegisterRoutes(RouteCollection routes) {
routes.MapPageRoute("Home", "", "~/Home.aspx");
routes.MapPageRoute("Posts", "Posts/{blog_url}", "~/blog-description.aspx");
}
protected void Application_Start(object sender, EventArgs e) {
RegisterRoutes( RouteTable.Routes);
}

Now this works fine on local  but when i move the same to live it shows error 404 .I want to create a log actually what url is called.