RouteConfig.cs

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

            routes.MapRoute(
                    "Sub", // Route name
                    "{controller}/{action}/{id}", // URL with parameters
                    new { controller = "SubdomainController", action = "AnyActionYouLike", id = UrlParameter.Optional },
                    new { controller = new SubdomainRouteConstraint() },
                    new[] { "MyProjectNameSpace.Controllers" }
                    );

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

SubdomainRouteConstraint.cs

public class SubdomainRouteConstraint : IRouteConstraint
    {
        public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values,
                          RouteDirection routeDirection)
        {
            string url = httpContext.Request.Headers["HOST"];
            int index = url.IndexOf(".", System.StringComparison.Ordinal);

            if (index < 0)
            {
                return false;
            }
            //This will bi not enough in real web. Because the domain names will end with ".com",".net"
            //so probably there will be a "." in url.So check if the sub is not "yourdomainname" or "www" at runtime.
            string sub = url.Split('.')[0];
            if (sub == "www" || sub == "yourdomainname" || sub == "mail")
            {
                return false;
            }

            //Add a custom parameter named "user". Anythink you like :)
            values.Add("subdomainAsAParameter", sub);
            return true;
        }
    }