C# .net 获取外网ip

public string GetIP()

    {

        string strUrl = "http://www.ip138.com/ip2city.asp"; //获得IP的网址了   

        Uri uri = new Uri(strUrl);

        WebRequest wr = WebRequest.Create(uri);

        Stream s = wr.GetResponse().GetResponseStream();

        StreamReader sr = new StreamReader(s, Encoding.Default);

        string all = sr.ReadToEnd(); //读取网站的数据   

        int i = all.IndexOf("[") + 1;

        int j = all.IndexOf("]");

          string ip = all.Substring(i, j-i).Trim();

return ip;

    }