Have you heard of a crossover ethernet cable? One end is normal the other has pins 1&2 switched with 3&6. This keeps the TX wires of 1 device connected to the RX of the other.
If you connect a computer to a switch, router or hub you use a straight through cable. If you connect a computer to computer or router to router, switch to router, switch to switch, you use a crossover cable. See DCE/DTE for more info on the 2 types.
The uplink port on a switch crosses the pins so you can use a straight through cable where you otherwise would have to use a crossover cable. (Sometimes a button that changes a port back and forth)
Got it?
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