private static readonly string[] _ipHeaderOrder = new[] { "HTTP_X_FORWARDED_FOR", "HTTP_X_CLUSTER_CLIENT_IP", "REMOTE_ADDR" };

      public static string ClientAddress(this HttpRequest request)
      {
         foreach (string header in _ipHeaderOrder)
         {
            string ipAddress = request.ServerVariables[header];
            if (ipAddress != null)
            {
               return ipAddress;
            }
         }
         return string.Empty;
      }