How to send a UDP packet to a specific computer when all the computer on the network have the same public IP address?

Question

Here's the problem, it's very simple (to understand..):

I have 2 computers at home, they both have the same public IP address (e.g. 1.2.3.4).

I have 1 computer at a coffee place (different network) so it has a different public IP address.

I want to send a message (e.g. "hi") from the computer at the coffee place to ONE of computers I have at home.

I'm using Java, think of the following very simple program for the sender (I took off exception handling for simplicity):

In main I do:

sendPacket("hi");

and I have

void sendPacket(String message){
    DatagramSocket myServerSocket = new DatagramSocket(9000);    // server socket
    byte[] sendData = new byte[message.length()];    // build msg
    sendData = message.getBytes();
    InetSocketAddress destSocketAddr = new InetSocketAddress("1.2.3.4", 9000);    // destination socket addr
    DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, destSocketAddr);     // make packet 
    myServerSocket.send(sendPacket);    // send packet
}

If I have my listener (receiver) running on both computers at home (both with the same public IP address 1.2.3.4) how can I specify which one I intend to send this message to?

Answer 1

If both of your home computers have the same public IP address, that means those computers are using NAT or Network Address Translation (strictly speaking, it's Port Address Translation or NAT Overload, but commonly referred to as just NAT).

What this means is that in order to initiate a connection from the outside to any of your machines inside NAT, a Port Forwarding must be set in your router (typically your modem), so that you map a specific port of your public home IP address to a specific private IP address inside your home.

Let's say you have computers A and B in your home like this:

             Router / Modem
              192.168.0.1 
                  ||
       ++=========++========++
       ||                   ||
  Computer A           Computer B
  192.168.0.2          192.168.0.3

Now, let's assume you need Computer A listening on TCP port 9000 (ports can mainly be TCP or UDP), you could forward public port 9000 directly to Computer A's 9000 port:

Forward TCP/UDP on public port 9000 to private port 9000 on 192.168.0.2

To send a message to computer A, just send it to 1.2.3.4:9000. But what if the other PC only listens on port 9000 too? You cannot also assign public port 9000 because it is taken by Computer A. You could do this:

Forward TCP/UDP on public port 9001 to private port 9000 on 192.168.0.3

This way, computer B still receives messages on port 9000, but they would need to be sent across the Internet to 1.2.3.4:9001. Your router's NAT automatically translates the port as the data packets enter (and leave!) your home network.

In the end, the sender would need to adjust the destination port in order to 'talk' to different machines behind NAT.

Hope this makes sense.