Which cable do you actually need?
| Cable | Both ends | You reach for it when… |
|---|---|---|
| Straight-through | Same standard (B–B or A–A) | Connecting unlike devices: PC to switch, switch to router. This is almost every cable you will ever make. |
| Crossover | One end A, the other B | Connecting like devices directly: switch to switch, PC to PC. Modern gear with auto-MDI/MDI-X usually handles this for you, so it is rarer than it used to be. |
Only pins 1, 2, 3 and 6 carry data on 10/100 Mbps links, which is why a crossover swaps exactly those two pairs. Gigabit uses all four pairs.
Learn the idea behind the game
Wire order is only half the story. Ethernet & MAC addresses explains what those pairs actually carry.