RJ45 isn't really RJ45

By stretch | Saturday, November 8, 2008 at 6:22 a.m. UTC
8P8C modular connector

Some interesting trivia for your Saturday: the ubiquitous network cabling plug best known as RJ45 has been living a lie. Technically speaking, the RJ45 designation actually refers to a registered jack, a telecommunications standard which defines both the mechanical interface (plug) and the wiring scheme used to terminate a cable. The modular connector itself is officially designated 8P8C (8 positions, 8 contacts), while the wiring standards used to terminate twisted-pair cabling are defined in TIA/EIA-568-A and B.

So why the mix-up? One can assume that the telco RJ nomenclature was subconsciously carried over to the network world by cable plant installers when twisted-pair LANs started gaining popularity in the late eighties and early nineties, and the terminology has simply stuck ever since.

About the Author

Jeremy Stretch is a network engineer living in the Raleigh-Durham, North Carolina area. He is known for his blog and cheat sheets here at Packet Life. You can reach him by email or follow him on Twitter.

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Comments


David Mofatt (guest)
June 20, 2011 at 6:32 a.m. UTC

So why does RJ45 have this weird switch of green and blue? IE why is the standard not white/orange, orange, white/green, green, white blue, blue, white brown, brown.

It would make wiring a lot easier.


Ryan Tucker (guest)
November 20, 2011 at 9:38 p.m. UTC

David: The color and white/color wires are twisted together as a pair within the cable, so from the cable manufacturing end of things, the scheme makes sense.

On the connector, things are a little more interesting. If you notice, the T568B standard (which is the standard I'm assuming you're using; there's two to choose from!) is

1 White/Orange
2 Orange
3 White/Green
4 Blue
5 White/Blue
6 Green
7 White/Brown
8 Brown

If we drop off the outer two pairs, we have:

3 White/Green
4 Blue
5 White/Blue
6 Green

This is compatible with two-line/four-conductor telephone connectors. Sure enough, you can plug a "RJ-11" or "RJ-14" connector into a "RJ-45" jack and it'll physically work. (I wouldn't make a habit of this.) This part of the scheme makes sense.

The part that doesn't immediately make sense is why it's not White/Brown, Orange, White/Green, Blue, White/Blue, Green, White/Orange, Brown -- in other words, why doesn't the pattern hold outside of the inner two pairs. Turns out that the spacing would be too great, and the twisted pair magic would be significantly impacted at high frequencies.

tl;dr: backwards compatibility, constrained by physics.

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