this post was submitted on 09 Apr 2024
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Imagine your computer is a big block of flats and your applications are all people who live in the building.
Mail sent to the building address alone isn't going to reach the intended recipient, because the postman doesn't know what flat to post it to. So they need additional information such as 'Flat 2C'
That's the basic concept of ports. It's basically additional addressing information to allow your computer to direct internet traffic to the correct applications.
When an application is actively listening on a port, it means that they are keeping an eye out for messages addressed to them, as designated by the port number. While an application is sending or receiving messages using a given port number, that port number is considered 'open'.
Now, all sorts of applications do all sorts of things. Some are for the public to use and there are some that are useful within trusted circles, but can be abused by malicious people if anyone in the world can send messages to it. Thus, we have a firewall, which acts as a gatekeeper. A firewall can 'block' a port, denying access to a given group of people, or 'unblock' it, allowing access.
VPNs are a totally different thing. They are literally middlemen for your internet traffic. Instead of directly posting a message to somewhere and receiving a direct reply back, imagine you flew out to Italy to use a post box there and receive replies from there.
I love your analogy for ports, but I'm not sure about the VPN one.
If you imagine network traffic as mail going through the postal system, then a VPN is like a private mail tunnel between two locations, that nobody else can enter or look into. Mail sent via the tunnel is private and nobody else can read it. The person at the other end of the tunnel can either open the mail themselves (ie a VPN from your laptop to your home server to access it when you're away), or forward the mail somewhere else (ie if you're routing Internet-bound traffic through it) and nobody will know it came from you originally.
I'm not sure that's a completely accurate analogy either. When you're using a VPN people can still see that you are sending traffic through your tunnel, they just can't tell what it is that you're sending. It's like looking through frosted glass; there's definitely something moving in there but you can't tell what.
I suppose the best way to describe it is you send a locked box to a trusted friend; everyone handling it can see the box but can't tell what's inside. Inside the box is a letter, your friend posts it so it looks like it came from them. Your friend then gets a reply, puts it in a locked box, and send it back to you. Nobody between you and your friend can snoop on your mail but anyone between your friend and the final destination can.
As soon as I read this I read the rest of your comment in Al Gore's voice, ca 2k SNL, lol.