Ethernet: Straight-through VS Crossover Cable

Straight-Though

    PC <=> Switch
Router <=> Switch

Crossover

    PC <=> Router
Router <=> Router
Switch <=> Switch

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Exceptions and notes:

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Consumer routers generally have one router port labeled [INTERNET] and four switch ports. If you want to connect a PC to any of the ports NOT labeled [INTERNET], you'd use a straight-through cable in that case since they're technically switch ports.

On almost all modern devices, you don't have to think about this topic since the network cards can auto-detect whether to transmit/receive in crossover mode or not.