always got the server ip instead of client external ip

i wanted to get the client ip, but after i upload to the server, it always get the server ip instead of the client's.
but if i directly visit http://checkip.dyndns.org , it do return my valid ip, here is the code: 
public string getExternalIp() { WebRequest request = WebRequest.Create("http://checkip.dyndns.org/"); WebResponse response = request.GetResponse(); Stream data = response.GetResponseStream(); string html = String.Empty; using (StreamReader sr = new StreamReader(data)) { html = sr.ReadToEnd(); } return html; } any advice? been working this for an hours...
...