Protocols and Ports

[00:00:00]
>> On top of these UDP and TCP circuits, we have this idea of protocols. Protocols are literal languages that we use to interact with different kinds of services. So a really common one on the internet is HTTP and HTTPS. HTTPS just has a layer of TLS encryption over HTTP.

[00:00:22]
To send emails we use SMTP, which we're gonna get into. It's actually a very simple protocol that's quite similar to HTTP in some ways. There's also IMAP and POP3, also text-based protocols that you can use to read email messages. There's IRC, which is a really old but still useful text-based chat protocol.

[00:00:45]
Then others like FTP, not used as much anymore, but SSH is still pretty common. So all of these different services, typically, have kind of like a default port. This is not always true. And some of them have several ports that they typically use. FTP is kind of a wild card here because it has a command port.

[00:01:09]
It's kind of designed a bit like how satellites work. Actually, it's pretty interesting because you have a port that you listen on, I think it's 21, or 20, or something, and that's the control port. And then, FTP will actually go ahead and open additional ports when it does a file transfer.

[00:01:27]
And so it's kind of complicated to set up a network config for it for that reason. But it's fascinating because that's also how, if you have a ground station and wanna talk to a satellite, you have usually a low but more reliable control frequency that you use. And then you use a higher frequency that has more throughput to pull down weather raster images from cameras that are pointed at the earth, that kind of thing, so.

[00:01:55]
Kind of a fun way to remember how FTP works, and also, how satellites work. So each of these different kinds of systems lives on a usual port. So these are numbers between 1 and 65,000 or so that differentiates the service on the system, for the most part. You can always run something on a different port.

[00:02:20]
And you have to do because on most systems to bind to a lower port, you have to have admin privileges or you have to have special kind of configuration. So it's pretty common when you're developing to run a HTTP server on port 5000 or port 8000 or something like that.

[00:02:40]
Here's how it was, 21 for the control port of FTP. SSH is usually 22, 25 for SMTP, 80 is HTTP, etc. IRC has got a bunch of them, but I think that's the most common one that's typically used. So here's something that you can all try on your system.

[00:03:04]
If you go ahead and if you have netcat installed, you can try nc- lp 1024, which probably auto works. So right here, when you run netcat with dash l, it listens on a port, and p tells it what port to listen on. So if you try to do 1023, though, it probably won't work.

[00:03:29]
So I'm gonna go ahead and do that on my screen. Whoa, whoa, whoa. [LAUGH] All right, so it's installing something. [LAUGH] So if I listen on 1024, there we go. It works because it's just waiting for connection. I'll get a little more into how to use netcat for testing things.

[00:03:51]
If I do 1023, permission denied. So this is a default kinda policy that most Unix C systems have. I don't know how it works with Windows, but it might be similar, it might not, who knows?