As far as I know there is no general way of doing this. It is one of the
issues addressed by UPnP. If your gateway supports UPnP that would
provide a mechanism, and a Java implementation is possible. Some people
think UPnP is a security risk and disable it, so unless you have
management authority over the LAN you may not be able to use this
approach even if the hardware supports it.

Mark Thornton

There are not even a 100% safe way.

You will need to ask an external server what IP it see.

Below are a small example using a danish service.

Arne

============================

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class ExternIP {
    public static String getIP() throws Exception {
        URLConnection uc = (new
URL("http://www.myip.dk/")).openConnection();
        BufferedReader br = new BufferedReader(new
InputStreamReader(uc.getInputStream()));
        StringBuilder sb = new StringBuilder("");
        String line;
        while((line = br.readLine()) != null) {
            sb.append(line);
        }
        br.close();
        Pattern p = Pattern.compile("(?:<title>Your IP:
)(\\d+\\.\\d+\\.\\d+\\.\\d+)(?:</title>)");
        Matcher m = p.matcher(sb.toString());
        m.find();
        return m.group(1);
    }
    public static void main(String[] args) throws Exception {
        System.out.println(ExternIP.getIP());
    }
}

Re: Getting A Computers IP Address

Why?  What does that do for the end user?*

Had you considered ..

<sscce>
import javax.swing.*;

class GetIPAddress {

 public static void main(String[] args) {
   JOptionPane.showMessageDialog(
     null,
     "The appl. needs you IP address so it can AAA\n" +
     "To find the IP address on your system, do BBB");
   String ipAddress = JOptionPane.showInputDialog(
     null,
     "IP address");
   System.out.println("IP Address: " + ipAddress);
 }
}
</sscce>

Where AAA is a text that is sufficiently convincing that
it would *motivate me, as an end user, to supply an IP
address, and BBB is OS specific instructions.

Easy done!

Have the server report the address of the endpoint of the connection.  This
will give you the NAT or proxy server address.
--
Mark Rafn    dagon@dagon.net    <http://www.dagon.net/>

On Sat, 08 Dec 2007 15:46:18 -0500, Hal Vaughan
<hal@thresholddigital.com> wrote, quoted or indirectly quoted someone
who said :

Usually internal IPs are masked by the firewall or proxy server.  For
the various IPs you can discover, see
http://mindprod.com/jgloss/ip.html