If you want to understand networking theory, the most important concept to understand is the OSI model (7-layer network model).
The OSI model ultimately does not map 1:1 with the Internet - the Internet uses a similar 4-layer model.
As a network engineer, the most important (OSI) layers to me are 1-4
It all seemed like "theoretical stuff that's not useful in the real world" when I was first taught this, but having worked with it, it's a crucial way to understand what's going on. You'll see lots of posts here that mention layer2 and layer3. Learning how to distinguish what layer a problem comes from is key in learning how to break a problem down into testable pieces.
If you don't see the MAC address of another LAN host in the ARP cache, for instance, then you know that the problem is local to your own local network segment - i.e. it's likely either a layer 2 problem (wrong vlan, perhaps?) or a layer 1 problem (cable between switches got unplugged). It could be layer 3 if the other host has the wrong IP settings configured... but it's certainly not going to involve port numbers or access rules in an HTTP server, so don't bother looking there if you don't get ARP replies.
When given a spoon,
you should not cling to your fork.
The soup will get cold.