Broadcast and point to point server in the same port

Question

I'm trying to set up a system where I can send point to point messages via sockets to certain ips, send broadcast messages to every pc in the network, and also be able to listen for point to point messages and broadcast messages, all in the same port. I've been able to establish a point to point connection using the Socket class to send the message, and using the ServerSocket class to listen for messages, but I can't seem to get broadcasting to work.
I read somewhere that for broadcasting one should use the DatagramSocket class. The problem is i'm not sure which ip i'm supposed to specify when I send the message. Also, do I need a special listener to listen for broadcast messages, or will the ServerSocket listener work for that?
This is what I have for the broadcasting part:
String msg = myId;
DatagramSocket sendSocket;
sendSocket = new DatagramSocket(Constants.COM_PORT); // 4567
InetAddress group = InetAddress.getByName(Constants.BROADCAST_ADDR); // "192.168.1.1"
DatagramPacket dp = new DatagramPacket(msg.getBytes(), msg.getBytes().length, group, Constants.COM_PORT);
sendSocket.send(dp);
sendSocket.close();
Thanks in advance
Diego

Answer 1

It worth mentioning that UDP/broadcast and robust do go together easily.
- UDP does not guarantee delivery. i.e. messages can be dropped and you won't be told. Lost packets are not retransmitted either.
- UDP can fragment packets if larger than 532 bytes long.
- UDP packets can arrive out of order (though I have never seen this myself)
If your receiver is under load/busy it can miss many packets if its buffers overflow.