Explain Like I'm Five

A port is like an apartment number. Except instead of apartments, it’s just mailboxes. Bob’s mailbox is number 25. If you want to get a message to Bob, uou write “box 25” on the address.

A port number is just an additional piece of info that the operating system associates with a particular program. If something comes in on port 22, the sshd is going to handle it because it’s “listening on port 22”, meaning that it has registered itself with the OS as the recipient of anything that comes in marked “port 22”.

Imagine your network card is a house. That house has an address (IP address in this case) and to get into the house you need doors (ports). The house is pretty big so it has exactly 65,535 doors (ports). But because the owner of the house is not insane, most of the doors are locked (firewall).

When something communicates with you (or you with something), you use a port. For example when you open a webpage, your browser sends a request to the server on port 80 or 443 (80 for http, 443 for https). Those are standardized ports which have a well-defined meaning, but in general you only need the client (browser, app, whatever) and server to use the same port for the service.

Let me know if that clears it up!

If an ip address is a house number, then a port is an apartment number.

Imagine you need to go see your doctor. They work in a building with 65535 rooms. Some rooms are empty. Some rooms have people in them that provide different services. But you need your doctor so you look up their location.

You learn the building address (IP address) and the room number (port)

In practice, you attach services to specific ports so that other computers can access those services. Typically, http traffic is on port 80 and https is on port 443. So if you visit a website, you are likely connected to a server on one of those two ports. But it’s not a requirement. You could create a website and put it on port 2097, or 532, or 47210; it doesn’t matter.