I've been asked a couple of times from my 'Live/Ethernet LED' tutorial in the hardware section (+FAQ) how do you run two ethernet connections down a single RJ45 cable..

This is extremly useful is you have taken the time to run a long RJ45 cable around your house, only to discover that you now should have run two cables as you need another one ... doh!

The 10/100 Mbit/spec only actually uses 4 of the 8 available wires as so :-

1. White/Orange (used - Tx Pair 1)
2. Orange/White (used - Rx Pair 1)
3. White/Green (used - Tx Pair 2)
4. Blue/White - not used.
5. White/Blue - not used.
6. Green/White - (used - Rx Pair 2)
7. White/Brown - not used.
8. Brown/White - not used.

So all we need to do is wire the 2 unused pairs (4 wires) into the same connections as you would have done normally in the 2nd RJ45 plug at both ends of the RJ45 wire - ie

(was 5 now 1) White/Blue (now used, Tx Pair 3)
(was 4 now 2) Blue/White (now used, Rx Pair 3)
(was 7 now 3) White/Brown (now used, Tx Pair 4)
(was 8 now 6) Brown/White (now used, Rx Pair 4)

There you have it, 2 Ethernet connections down a single RJ45 wire.

There are a few different ways to practically implement this

A.) Use a RJ45 faceplates, where you can simply wire the 'fixed' cabling as above, keeping the RJ flyleads the same. You'll need to buy 2 x RJ45 dual faceplates but it will look much more professional ...
B.) Buy some RJ45 plugs and an RJ45 crimper - not the most cost effective option if you are just doing a couple of connections though ...
C.) Lastly, cut the ends off existing RJ45 patch cables and connect the wires together. It's important to try and keep the 'twists' going, especially for longer runs of cable. Ideally solder the wires together and use heat shrink, but if you're really on a budget, then electrical tape and twisted wires should work ok ..

Last couple of points -

1. This will NOT work for Gigabit Ethernet - that uses all 4 pairs.
2. Make sure you identify the pins correctly, the colour codes I've given 'may' not be valid for your cables (there are two different standards.. )

Look here to check the RJ45 Pinout (courtesy datalinkcom.net)

Enjoy ...

Why wouldn't you just get a switch? Seems like it would be easier and its about $20

because you've already gone through the trouble to wire up the wall, and this looks SO much nicer also you don't have to hog up a power outlet with a switch if you do it this way.

Absolutely ..

Here's a quick diagram too for those that follow diagrams better than my explanation ...

Very helpful, RBJTech. Thank you for posting that.

just out of curiosity, does this mess with the 100M spec? or could one still use a 100M piece of ca5e/cat6 and not have any issues?

also, what about GbE? i have one piece of cat5e running into the master bedroom where my 360 is and obviously 10/100 is fine for the 360, but i also have a laptop, and i move a lot of data around the condo, so GbE is kind of a must if i am going to use a wired connection, otherwise i would just the laptopls 802.11g connection.

thanks in advance for this and also your lan activity mod - one i am going to do since i plan on getting a new faceplace anyway in case i mess it up, but i have always been curious with lan activity, any chance for a hdd activity led??

thanks again for your hard work and knowledge

thanks, skimmed right over that.....

Just a minor correction, pins 1 and 2 are both used for transmit, pin 1 for +, pin 2 for -, and the same for pins 3 and 6 as receive.

Just one thing to bear in mind with this! From memory Cat5e is rated at about 125Mbps, whereas Cat6 is rated at about 350Mbps.

What this means with Cat5e is that you can't run two 100Mbps links down the same wire as it will exceed the specification, and result in a lot of lost data packets - which in turn means very slow network speeds. When I've done this in the past, I've always ensured that one of the links runs at 100Mbps, and the other at 10Mbps.

If you're using Cat6 then you can run two links at 100Mbps.

It's worth checking that your cable is Cat5e, and not Cat5 - Cat5 is only rated at 100Mbps, and therefore you can only really get away with both links running at 10Mbps.

I have no idea why this was made a sticky, however as mentioned, while it certainly can be done, you'll be very likely to experience crosstalk. We used to run voice and data on the same category 5 cable, and again, while it can be done, you'll be losing performance on the link.

Do yourself a favor and buy a switch. I build data centers for a living, and if it was a recommended way of doing things, cable costs for new sites would have been reduced by 50%.

Take off the sticky, it's not worthy.

First off - I never asked for it to be made a sticky... It's just useful info.

Secondly, I have never had ANY packet loss issues running two 100 Mbit connections down a single RJ45 cable (cat 5e). As long as you maintain the twists and keep to the correct pairs then this is actually no different whatsoever from a packed bundle of UTP RJ45 cables - I also build data centres ...

Thirdly - we are not talking data centre grade communications here - I'd be very much surpised if both channels will be anywhere near maxed out at the same time in a typical home environment. I would not do it in a commercial implementation but for home use it's perfectly acceptable IMO.

Lastly - the 100Mbit or even 10Mbit/Sec speed is not the bottleneck on most peoples broadband/Xbox Live connection - it's the broadband itself, so speed is not really the issue here, it's practicality ...

If you don't want to do it fine - but I assure you it works fine for 99.9% of the people on this board ...

I've done this for years, although it is a halved for a PC and XBox - they both won't be maxing out their bandwidth at the same time. I don't think there is any problems of interference - the 4 pairs are twisted for the full length of the cable, and seperated at the double wall socket at either end. Just the same as gigabit ethernet uses, except the data is not combined into one line.

great tut. thanks

awesome tut. worked great. thankx!!

People, this 100 Megabit thing is a Red Hearing. You don't need to worry about 'combined bandwidth'. If that was the case, you wouldn't be able to Transmit at 100 Megabit while Receiving on the other differential pair at 100 Megabit rates.

If you don't want a switch (and it's latency) between you and your ISP, this will work just fine.

Id actually ran a Cat5e cable down the outer wall of my house, along with some other cables, the missus didnt want wide ducting, and my ducting is full. Glad I spotted this. It isnt brilliant for very long runs of cable due to cross-talk, and people who want GbE will have to run another cable... But this will work perfectly for people like me.

Just a small change though, there is only 1 wiring standard for straight through ethernet, T568A. The other one is T568B, and it is the same but with a reversed pair, for crossover connections.