Question

I have both a client and server application using UDP port 25565.
In order to run these on the same machine, because only one application may bind itself to port 25565, does this mean that it is necessary for me to use two separate ports for transmitting data between the applications?

What I have in mind is the following –

Client -> 25565 -> Server

Client <- 25566 <- Server

Is this the only solution or is there another way of handling this?

Answer 1

• Your server application open a port and wait for client to connect.
• Client need to know this port in advance so it can establish a connection to the desired service.
• Client can use any available ports to initiate this connection (better to use ports > 1000).
• The server sees in the incomming packet wich port the client is using, so it will send anwser to it. No need to specify it in your design.
• After handshake the TCP/IP connection is then identified by these 4 values : server IP, server port, client IP, client port.
• No other connection could have the same four values.

To answer your question. A TCP/IP connection is bi-directional, once established, the server can send data to the client and the other way around.

I would draw the scheme like this :

SERVER port 25565 <-> CLIENT port 25566 (or any other port)