我是用路由器上网，如何获取这个外网ip

Question

public IPAddress GetIP()
        {
            WebClient client = new WebClient();
            byte[] bytRecv = client.DownloadData("http://www.ip138.com/"); //下載數據
            string str = System.Text.Encoding.GetEncoding("gb2312").GetString(bytRecv);
            string r = @"(((\d{1,3})|(1\d{2})|(2[0-4]\d)|(25[0-5]))\.){3}((\d{1,2})|(1\d{2})|(2[0-4]\d)|(25[0-5]))";
            string ip = System.Text.RegularExpressions.Regex.Match(str, r).ToString();    //提取信息
            return IPAddress.Parse(ip);
        }