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We have 8-wired ethernet cable in the wall and wall socket for two ethernet cables. I am facing the decision how to connect the wires.

I know about two basic possibilities:

  1. I can use the cable as Gigabit ethernet and use it with only one PC. The other connector in the wall wouldn't be usable.
  2. I can use first 4 wires as one 100 megabit ethernet connection and use the other 4 wires as second 100 megabit connection.

My question is: Can I somehow achieve the combination of both possibilities?

I don't want to decide right now what setup I need; I would like to decide each time I plug computers to the wall which setup is better.

Suggested partial solution:

All eight wires will be one gigabit connection, but 4 of them will be also connected to the second half of socket.

I am the only user. And I will remember that in order to connect two computers with 100Mbit connection I need to plug the computers to the wall with only 4 wired cables. This will ensure that one computer won't receive signal from the other computer on unused wires.

However I would like not to do the same on the other side of cables. Hence I would like to have two 100Mbit connection, each using 4 wires, but the ethernet cable in the router from one connection will have nonsense on the unused wires. Is this going to work?

It might depend on the order of confusingly connected wires. Can you suggest correct wiring?

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  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 12, 2017 at 4:38

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My suggestion would be to wire the socket as a single connection and then if you later decide to use it as two cables, just use a splitter (a little piece of cable with one plug and two sockets) at both ends. Just make sure to get a splitter for Ethernet (which connects two pairs to one socket and two to another) and not ISDN (which just connects both sockets in parallel).

Connecting the wires to two sockets in parallel may cause signal problems for the gigabit connection and you (or someone else) will forget to use it correctly at least once and plug one end of the cable as a single connection while connecting two computers at the other end.

Upd.

Internal wiring of ethernet splitter

enter image description here

Splitter kind 1, kind 2 useful for patchpanel

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One cable for one connection is the correct way to go. Two 100 Mbps connections on one UTP wire might work under some circumstances, but it might as well cause all kinds of weird problems.

If the cable run is straight and not too long, you could try to use the existing wire to pull through a pair of cables and use both sockets. Careful, this could also go wrong and you could end up with no working cable at all.

If you want to connect multiple devices, the best option might be to wire one socket and then set up an ethernet switch to provide multiple ports.

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    Two 100Mbit connections on one cable works just fine - it the pre-Gigabit era that was dirt common, and we never saw any "weird problems" from it. Consider than a gigabit cable is carrying 4 streams at 250MHz on the same cable. But a single gigabit cable and a gigabit switch at the far end is the right approach to it in the current era. It's at least 5 times as fast, for one thing. It's not limited to two ports, for another.
    – Ecnerwal
    Aug 18, 2015 at 2:11
  • @Ecnerwal, currently working with an edu that did this in some of their dorms. At 100Base-TX they typically start getting errors on the switch interfaces and as such limit the speeds to 10Base-T (where they effectively have 0 errors) to avoid this. I would call higher than normal errors a potential source of "weird problems." Just because it sometimes works or sometimes doesn't get noticed doesn't mean that it will work well in all circumstances.
    – YLearn
    Apr 5, 2016 at 5:41
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The ethernet UTP cabling standard defines that a single UTP cable (for a single connection) use four pairs (8 wires). If you have an 8-wire cable, you have a single UTP ethernet connection. Anything else is non-standard.

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Everything you posted is possible, except for this:

However I would like not to do the same on the other side of cables. Hence I would like to have two 100Mbit connection, but the ethernet cable in the router from one connection will have nonsense on the unused wires

It is possible to split a single 8-wire UTP cable into two 100Mbps cables that only require 4-wires. But then both sides of the two wires would require two router ports.

Each of the 8-wires are grouped together into 4-pairs:

enter image description here

For the Gigabit standard (1000BASE-T), all four pairs are used. And data can be sent/received on wire pin position 1 - 8.

For the FastEthernet standard (100BASE-TX), only pairs 2 and 3 are used, and data can be sent/received on wire pin positions 1, 2, 3, or 6.

You can take one UTP cable with all 8 pairs, and split them out into two 8P8C connectors (also commonly referred to as RJ-45 jacks) that only use pairs 2 and 3. But on the other side, you could only connect one of the split-UTP cables to the correct Pair 2 and Pair 3.

You wouldn't be able use Pairs 2&3 on one side of the cable and Pairs 1&4 on the other. Those pin positions are essentially disabled when your NIC speed is set to 100Mbps.

The other side would also have to use two different 8P8C connectors.

That said. I think your simplest solution is to just use a switch. Then your two wall ports can be extended into as many switch ports as necessary, and then on the other side you would still only need two router ports.

Even a simple hub will do if you're just trying to get one or two extra ports out of your already existing wall-run.

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  • The ANSI/TIA/EIA 568 standard requires that a four-pair cable be terminated to an eight position connector, and it specifically disallows splitting it behind the faceplate; any adaptation must be made outside the faceplate. Using the single cable for multiple applications is non-standard, and, as @YLearn points out, your mileage may vary, which is why the standard does not allow this.
    – Ron Maupin ♦
    Apr 5, 2016 at 15:20
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    Agreed, @RonMaupin, but this question isn't asking about the standard.
    – Eddie
    Apr 5, 2016 at 15:23
  • I don't think we should be recommending things that violate a standard, and may, or may not, work correctly. If it works for this OP, and someone researches it and find the question and answers, and it doesn't work for that person, have we really served the community?
    – Ron Maupin ♦
    Apr 5, 2016 at 15:27
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    @RonMaupin Again, the question was "is this possible", not "is this a best practice". I'm not recommending they do it, in fact I'm telling them why it won't work with their set up. Hit me up in chat if you want to continue, but I don't think this back and forth is really adding anything to the answer.
    – Eddie
    Apr 5, 2016 at 15:51
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You can use a splitter (you can even buy these abominations for both 2 FastEthernet links in one 4 pair cable, or FastEthernet and POTS) at both ends of the permanent link of the run but you'll get a lot of crosstalk depending on the cable type used for the run and how long it is. I've never seen an application where these splitters work as advertised, usually you get so much errors when both are in use that it renders one link unusable.

On a side note, any particular reason for why you just can't install a switch at the far end of the link?

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