通过InterNet，获取本机访问外网IP

public static string GetIPFromInternet()
        {
            string strUrl = "http://iframe.ip138.com/ic.asp";
            Uri uri = new Uri(strUrl);
            WebRequest webreq = WebRequest.Create(uri);
            Stream s = webreq.GetResponse().GetResponseStream();
            StreamReader sr = new StreamReader(s, Encoding.Default);
            string all = sr.ReadToEnd().Trim();
            int i = all.IndexOf("[")+1;
            int m = all.IndexOf("]", i);
            //分析字符串得到IP   
            string ip = all.Substring(i, m-i).Trim();
            return ip;
        }