A multicast demonstration. Please run from several machines in the same subnet. Instructions inside file due to crappy GIST description formatting.

Node.java

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.net.UnknownHostException;

/**
 * To run:
 * javac Node.java && java Node; 
 * 
 * Sample output:
 * node: A8-20-66-55-EF-F3 sending message: A8-20-66-55-EF-F3---0
 * node: A8-20-66-55-EF-F3 received: A8-20-66-55-EF-F3---0
 */ 
public class Node extends Thread {

    	private String id;
	private MulticastSocket s;
	private InetAddress group;
	private int port;

	//The Multicast Group we wish to join:
	private static final String MULTICAST_GROUP = "228.5.6.7";

	public Node (String id, int port ) throws IOException{
		this.id = id;
		this.group = InetAddress.getByName(MULTICAST_GROUP);
		this.s = new MulticastSocket(port);
		this.s.joinGroup(group);
		this.port = port;
	}

	public void send(String msg ) throws IOException{
		msg = id + "---"+msg;
		//Send our MAC plus our unique count.
		System.out.println("node: "+id + " sending message: "+msg);
		DatagramPacket hi = new DatagramPacket(msg.getBytes(), msg.length(), group, port);
		s.send(hi);
	}

	/**
	 * @param args
	 * @throws IOException 
	 */
	public static void main(String[] args) throws IOException {

		String id = getMyId();
		int portt = 10000;
		Node n = new Node(id, portt);
		n.start();//start receiver

		//sending messages forever
		int count = 0;
		while (true){
			try {
				n.send(""+count++);
				Thread.sleep(10000);
			} catch (IOException e) {
				e.printStackTrace();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}

	}

	/**
	 * @return the Mac address as a string
	 * @throws UnknownHostException
	 * @throws SocketException
	 */
	private static String getMyId() throws UnknownHostException,
	SocketException {
		InetAddress ip = InetAddress.getLocalHost();
		NetworkInterface network = NetworkInterface.getByInetAddress(ip);
		byte[] ha = network.getHardwareAddress(); 
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < ha.length; i++) {
			sb.append(String.format("%02X%s", ha[i], (i < ha.length - 1) ? "-" : ""));		
		}
		return sb.toString();
	}

	@Override
	public void run(){
		while (true){
			//Just to be sure we get some new buf. too used to C and it giving
			//you the same uncleared memory arrays.
			byte[] buf = new byte[1000];
			DatagramPacket recv = new DatagramPacket(buf, buf.length);
			try {
				s.receive(recv);
				System.out.println("node: "+id+ " received: "+new String(recv.getData()));
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	public void shutdown() throws IOException{
		s.leaveGroup(group);
	}

}