That's a good question! When you use nc -l 12345, you are explicitly telling Netcat to choose and use port 12345.
It's not that Netcat "chooses" a port randomly or automatically in this scenario. Instead, you, as the user, are instructing it to listen on a specific, designated port that you provide (12345 in this case).
Think of it like this:
- Imagine you're setting up a small shop. You decide it will be shop number 12345.
- When you use
nc -l 12345, you're essentially putting up a sign outside your shop that says "Open for business at Shop #12345!"
If you didn't specify a port, Netcat wouldn't know where to listen. The -l flag requires a port number to be specified so it knows which "door" to wait at for incoming connections.
We chose 12345 because it's typically unused by common system services and is easy to remember, making it a good choice for learning and experimentation.
