RJ45 Ethernet Pinout
T568B wiring, looking at the contacts with the clip on the far side. Check your board against its silkscreen before wiring power.
T568B or T568A
Both standards work identically; B is what most patch cables and most of North America use, and A simply swaps the orange and green pairs. The only rule that matters is using the same standard at both ends, unless you deliberately want an old-style crossover cable.
TX and RX depend on the end
At 100 Mbps a network card transmits on pins 1 and 2 and receives on 3 and 6. A switch port does the opposite, which is what the X in MDI-X means. At gigabit speeds all four pairs carry data in both directions, so the labels stop meaning much at all.
Crimping sanity check
Hold both plugs side by side, clips away from you, contacts up. The colors should read identically left to right. Keep each pair twisted right up to the plug, or gigabit will quietly refuse to negotiate.
Sources and datasheets
This diagram was cross-checked against the manufacturer documentation below plus at least two independent references before publishing. When they disagreed, the manufacturer won.
More boards and plugs live on the pinouts index.