public static string GetClientIPv4Address()
    {
      string ipv4 = String.Empty;
      foreach (IPAddress ip in Dns.GetHostAddresses(GetClientIP()))
      {
        if (ip.AddressFamily.ToString() == "InterNetwork")
        {
          ipv4 = ip.ToString();
          break;
        }
      }
      if (ipv4 != String.Empty)
      {
        return ipv4;
      }
      // 利用 Dns.GetHostEntry 方法，由獲取的 IPv6 位址反查 DNS 紀錄，
      // 再逐一判斷何者為 IPv4 協(xié)議，即可轉為 IPv4 位址。
      foreach (IPAddress ip in Dns.GetHostEntry(GetClientIP()).AddressList)
      //foreach (IPAddress ip in Dns.GetHostAddresses(Dns.GetHostName()))
      {
        if (ip.AddressFamily.ToString() == "InterNetwork")
        {
          ipv4 = ip.ToString();
          break;
        }
      }
      return ipv4;
    }

    public static string GetClientIP()
    {
      if (null == HttpContext.Current.Request.ServerVariables["HTTP_VIA"])
      {
        return HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
      }
      else
      {
        return HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
      }
    }