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RJ45 Pinout & Cable Reference
The T568A and T568B wiring order for RJ45 connectors, straight through against crossover, and the maximum run length and speed for each cable category.
Pin order, looking at the clip side down with the pins facing you
T568B most common
T568A
Cable category, speed and distance
| Category | Speed | Max run | Bandwidth | Notes |
|---|---|---|---|---|
| Cat5e | 1 Gbps | 100 m | 100 MHz | 2.5 Gbps up to 100 m in practice |
| Cat6 | 1 Gbps | 100 m | 250 MHz | 10 Gbps but only to 55 m |
| Cat6a | 10 Gbps | 100 m | 500 MHz | The practical choice for new 10 Gbps work |
| Cat7 | 10 Gbps | 100 m | 600 MHz | Shielded, uses non standard connectors |
| Cat8 | 40 Gbps | 30 m | 2000 MHz | Data centre only, short runs between racks |
Straight through, crossover, and why it stopped mattering
A straight through cable uses the same standard on both ends, which is what you want in almost every case. A crossover uses T568A on one end and T568B on the other, and it was once needed to connect two computers or two switches directly. Modern equipment detects and corrects this automatically, so unless you are working with old hardware you will never need one.
Pick one standard and use it across the whole site. T568B is the more common choice. The two are electrically identical and the difference is only which pairs sit on which pins, but a building wired half in each creates faults that are maddening to trace.
The hundred metre limit is the total channel, not just the long cable. Patch leads at both ends count toward it, so ninety metres of permanent cable plus two five metre patch leads is already at the limit. Beyond that, run fiber rather than hoping it works.