Open port

[1] Ports are an integral part of the Internet's communication model — they are the channel through which applications on the client computer can reach the software on the server.

Services, such as web pages or FTP, require their respective ports to be "open" on the server in order to be publicly reachable.

Packets directed at a port which the firewall is configured to "close" will simply be dropped in transit, as though they never existed.

Some malicious software acts as a service, waiting for connections from a remote attacker in order to give them information or control over the machine.

It is common security practice to close unused ports in personal computers, so as to block public access to any services which might be running on the computer without the user's knowledge, whether due to legitimate services being misconfigured, or the presence of malicious software.