You’re on the network, surfing away on the web, and there’s data just flowing to your box.  Even right now, when you’re looking at this web page, there’s data heading to your system. It’s network traffic and if you think of it as a pipe, your system has that flowing in all the time.  Imagine it as a water pipe, like this picture:

Leaky Pipe

We’ll pretend our pipe isn’t leaking, since that would mean we’re losing network traffic.  Our pipe brings us all kinds of traffic as we surf away, some of which we want and some of which we don’t.

How does the computer know what kind of traffic the pipe is bringing it?  Computers are mostly stupid, if you put legs on one and told it to walk off a cliff, it would happily do that. We have to tell the computer what kind of traffic to expect, either DNS or web traffic or email, or other kinds of traffic we’re not mentioning here.

Back to our pipe analogy because it’s a really useful way to think about the traffic. If before the pipe connects to us, it separates itself into different types of traffic, that would work, right?

Many Pipes

We can attach those little pipes to the pipe in the first picture and that would separate out the traffic.

The way modern operating systems (Linux© included) handle this is with ports.  The pipe connecting you to the network has labeled ports and the different kinds of traffic go to the different ports.  Web traffic goes to port 80, e-mail to port 25, DNS to port 53, and there’s more. Each machine has 65,535 ports on it for traffic to use.

I know you’re asking the next question:  How does the computer know what kinds of port do what kind of traffic?  Well, there’s a file for that.   It’s called /etc/services.  If you more it, you can find see people reserve ports for different kinds of applications.

There’s also a website  that keeps track of assigned ports.

Next time we’ll talk about what tcp and udp in the /etc/services file mean.