What is a port?
A number from 0 to 65535 that points to a specific program or service on a machine.
One server can run a website, a database and email, all on one IP address. So how does each message reach the right program? Ports.
What it is
A port is a number, from 0 to 65,535, that points to a specific program or service on a machine.
How it works
If the IP address is the building, the port is the apartment number. A program listens on a port, and every connection names one. Plain web traffic uses port 80, and HTTPS uses port 443. Run an app on your laptop, and it's often on localhost, port 3000.
Why it matters
Firewalls open and close ports to control what can reach a machine. And the error "address already in use" means another program got to that port first.
