MVC :: Subdomain Route Link Helper

I've created a new Route class, named SubdomainToAreaRoute, inherited from System.Web.Routing.Route, to handle subdomain.

The next implementation needed is to create a helper method to generate subdomain link.

For example:

Html.ActionLink("subdomain1", "account", "index") would generate:

http://subdomain1.example.com/account/index A series of other overloaded methods will also be implemented.

I undestand the geist of creating extension methods, which is one of the requirement to implementing these helper methods. Thus the main concern here is knowing what objects/classes to access and manipulate, and recommended algorithm to use.

There are some things that is unclear to me. For example, we register routes in the RouteTable.Routes (or simply route table), and whenever a url is requested, the url request is checked in the route table. If no url pattern matches, or if there is a matching pattern and the resource (e.g. web page) is missing, we receive a http error.

What is unclear here is in cases where specific physical path to a resource, like an image or javascript file is used for request. If these physical path url do not match the url patterns in the route table, how does an MVC application know to check for web application directory for the resource as the next step? As oppose to throwing an http error saying the resource can not be found.

This question may or may not affect my main concern here. But it is good to know if someone can explain it.

Focusing on registered routes, consider the following url route registration:
routes.Add("Home", new SubdomainToArea("", new { area = "Root", controller = "Home", action = "Index" } ) );
// route pattern 1
// intended to map to url: http://{area}.localhost.com
routes.Add("Default", new SubdomainToArea("{controller}/{action}", new { area = "Root", controller = "Home", action = "Index" } ) );
// route pattern 2
// intended to map to url: http://{area}.localhost.com/{controller}/{action}
routes.Add("Images", new SubdomainToArea("images/{*pathInfo}", new { area = "Root", controller = "Home", action = "Index" } ) );
// route pattern 3
// intended to map to url: http://{area}.localhost.com/images/{random path string}

How do I create helper methods to generate the appropriate url that matches these patterns?

I understand that extension methods for the UrlHelper class should be implemented to generate these url, and these url will be use in HtmlHelper class to generate the appropriate link tags.

In these helper methods, e.g. in the UrlHelper class, if passed a set of route values (such as controllers, action, areas and other url path details), how do we choose the appropriate route pattern that is registered in the route table, and then apply the route values to the pattern to generate the url?

How should I handle in cases where I don't specify one? What does the standard Html- Url- helper methods handle in case where no route name is not specified?

I understand the placeholder {controller}, {action} and {area} have special meaning and can be more easily identified and mapped to url pattern and so forth. How do we deal with custom placeholders we dynamically create, and use them for generating url?