This is the very basic description I gave in another forum recently:
Standard ethernet uses two pairs of wires, one send pair (wires 1 and 2) and one receive pair (wires 3 and 6). In order for Computer A to communicate with Computer B, those pairs have to be reversed (so that the send pair on Computer A connects to the receive pair on Computer B, therefore allowing Computer B to receive data sent by Computer A). Standard Ethernet cables are essentially straight through, which means that if you were to connect the cable between two computers, they won't be able to communicate.
A hub or switch port automatically reverses the pairs so that the hub or switch can communicate with the PC on the port, which is why often ports will be listed as MDIX (medium dependent interface crossover) ports, the X representing the crossover... i.e., it exchanges the pairs. The uplink, or MDI port, does not. When you connect two hubs or switches together, if you plug both ends of a standard cable into an MDIX port, you end up cancelling out the crossover. This is why one end needs to be in an uplink, or MDI port (non-crossover port).
This also explains why, for those doing the cheap 2 system network, you can take a crossover cable (pair 1,2 on one end is wired to pair 3,6 respectively on the other, and 3,6 is wired to 1,2) and plug the two systems directly into each other without any hub or switch.
As for where to get the straight through cable, pretty much any computer store will have a standard ethernet cable. If you must make your own (i.e., running through walls or something,) do a search on Google for "ethernet wiring" for some pages on the pin out. They are actually pretty simple to make.
Hmm... in my experience cable modems are setup to go from PC to Modem (direct connection) with a standard cable. So, if you introduce a hub, you would need a crossover to the modem and a standard to each pc.
Or, atleast thats how my modem works. Perhaps newer models are different now that home LANs are much much more common then when I signed up for broadband years ago.
It depends on the modem. I know my DSL modem actually has a button to press on the back so you can use either type and just press it in if your using a straight through. I am not sure about cable modems, but its possible. Otherwise typically you would require a cross over from the modem to the hub.
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