A class to detect the subdomain and pass it through as a route parameter

SubdomainRoute.cs

public class SubdomainRoute : RouteBase
{
    public override RouteData GetRouteData(HttpContextBase httpContext)
    {
        if (httpContext.Request == null || httpContext.Request.Url == null)
        {
            return null;
        }

        var host = httpContext.Request.Url.Host;
        var index = host.IndexOf(".");
        string[] segments = httpContext.Request.Url.PathAndQuery.TrimStart('/').Split('/');

        if (index < 0)
        {
            return null;
        }

        var subdomain = host.Substring(0, index);
        string[] blacklist = { "www", "yourdomain", "mail" };

        if (blacklist.Contains(subdomain))
        {
            return null;
        }

        string controller = (segments.Length > 0) ? segments[0] : "Home";
        string action = (segments.Length > 1) ? segments[1] : "Index";

        var routeData = new RouteData(this, new MvcRouteHandler());
        routeData.Values.Add("controller", controller); //Goes to the relevant Controller  class
        routeData.Values.Add("action", action); //Goes to the relevant action method on the specified Controller
        routeData.Values.Add("subdomain", subdomain); //pass subdomain as argument to action method
        return routeData;
    }

    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        //Implement your formating Url formating here
        return null;
    }
}