That's a very insightful question!
If the client tries to connect to a different port than the one the Netcat listener is on, the connection will fail.
Here's why:
-
The Listener's Role: When you run
nc -l 12345in the first terminal, you're telling that specific Netcat instance to exclusively listen for incoming connections on port 12345. It's like having a specific phone number (port 12345) and waiting for a call on that number. -
The Client's Attempt: If your second terminal (the client) tries to connect using
nc localhost 54321(trying to connect to port 54321, for example), it's essentially dialing a different phone number. -
No One Answering: Since there's no program or service listening on port 54321 (only your Netcat listener is on 12345), the client won't find anything to connect to. It will likely just hang, and eventually time out with a "Connection refused" or similar error message, because there's no server process assigned to that port to accept the connection.
Effectively, for two applications to communicate using TCP/IP (which Netcat does by default), they both need to agree on which port to use, and one side (the server/listener) must be actively listening on that port, while the other side (the client) initiates the connection to that specific port.
Experimenting with this is a great way to understand it! You could try running nc localhost 54321 in your second terminal without having a Netcat listener on port 54321 in another terminal, and see what happens.
