Can anyone explain exactly what a network segment is?
For example, is my PC a network segment? And my modem or router?
According to Wikipedia:
So your PC and router are on the same network segment.
A long time ago, when 10Mbps Ethernet was all the rage, you had thicknet and thinnet cable. All devices on the network had to be physically connected to a single cable. The Wikipedia articles of both "thicknet" and "thinnet" have great pictures on what the cables looked like.
So, that single cable connecting all devices was a segment. Since all devices were listening and transmitting to the same cable (a.k.a. media), any device could easily speak to any other device. If you wanted to speak to a device on a different cable (i.e. network), one of the devices on that cable needed to be a router and would forward your traffic back and forth.
Cable lengths had maximum lengths they could sustain before the signal would get degraded. But you could attach repeaters that would refresh the signal at that point and essentially extend the length of the cable, and thus the segment.
Now, we have switches. So there is no longer a direct electrical connection between devices anymore. Each device plugs into a switch and the switch intelligently repeats traffic where it is needed. But logically, it is as though all the devices are still speaking on the same cable back in thick/thinnet days. Just as in the case of the repeater above, the devices trying to talk "through" the switch don't know or care that it is there. So a switch can be said to extend the segment in the same way a repeater does.
Segment really refers to the actual network inbetween devices and not the devices themselves. PCs, modems, routers are devices "attached to" a segment.
Think of it this way. If a coworker was nearby, you can just shout to him and communicate to him. You don't need his phone number or need to know where he is. He's part of your 'network segment`.
Now lets say you need to use the phone to talk to another coworker, but he's not within shouting distance. You need to pick up the phone and dial his number. He's not in your 'network segment' because you need to use an intermediary device, the phone, to talk to him. The phone translates your voice to an electronic signal that then is decoded on his side.
While I'm not a network admin(they have their own strange lingo and rightly so), I hardly ever hear anyone use this term. More often, they use more IT specific terms like broadcast domain, collision domain, border area, etc etc.
Network segment might be more of a communications term.