This program runs a server that give you the IP address of every client that tries to connect. It works on both side (display the IP and send it back). It is quite useful as a whatismyip service on a local network.

Main.java

package fr.stackr.java.gimmeyourip;

import java.io.IOException;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.WriteAbortedException;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * This program runs a server that give you the IP address of every client that
 * try to connect. It works on both side (display the IP and send it back). Its
 * quite useful as a whatismyip service on a local network.
 * 
 * @author Romain Bochet
 */
public class Main {

	/**
	 * Translation of IP in ascii
	 */
	private final static int PORT_NUMBER = 7380;

	/**
	 * Endless while waiting for new connections.
	 * 
	 * @param args
	 *            Not used
	 * @throws IOException
	 *             If the socket initialisation fails
	 */
	public static void main(String[] args) throws IOException {
		System.out.println("GimmeMyIP started (host: "
				+ InetAddress.getLocalHost() + ") -- listen on port "
				+ Main.PORT_NUMBER);
		System.out.println("Command: telnet "
				+ InetAddress.getLocalHost().getHostAddress() + " "
				+ Main.PORT_NUMBER);

		ServerSocket srv = new ServerSocket(Main.PORT_NUMBER);
		while (true) {
			Socket socket = srv.accept();
			System.out.println("Client connected: " + socket.getInetAddress());
			OutputStreamWriter output = new OutputStreamWriter(
					socket.getOutputStream());
			output.write(socket.getInetAddress().getHostAddress());
			output.write('\n');
			output.flush();
			socket.close();
		}
	}
}