WHEN YOU HAVE A NETWORK SERVICE INSTALLED AND LISTENING ON AN EXTERNAL INTERFACE, you are allowing other systems to connect to you. If you use common services like web servers, SSH servers, and file-sharing servers, you are guaranteed to have well-known ports open. When you connect to a web server, for example, there is an expectation that it will be listening on port 80 or, more commonly today, port 443 for Transport Layer Security (TLS)-based communications (meaning encrypted communications). You can certainly have your web server listening on a different port, but that ...
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