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I have tried splitting the cables that go into the router with a Y and it does not work.

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    Please note that modern network devices can use all eight wires of the connection, so splitting the cables is done at the expense of bandwidth, if it works at all. Commented Jul 29, 2019 at 12:11
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    Phuhh, what sort of a splitter, exactly? The question doesn't say, and the answers to the linked question (Will an Ethernet splitter work?) also discuss different kinds of splitters... (There's one answer there that assumes all 4 pairs just forked, and another that assumes something that makes use of the two pairs not used by 100Base-TX.)
    – ilkkachu
    Commented Jul 29, 2019 at 18:41
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    The answer to your question in the title is actually rather simple: "Why do they sell Cat 5 Ethernet splitters if you can’t split the signal?" – Because people buy them. There's really not much more to it. Why do they sell oxygen-free gold-plated Ethernet cables that make your streaming audio sound fuller and richer? Commented Jul 29, 2019 at 19:48
  • Can you give details about the splitter you actually used, and how you used it? There are many different types of splitters, and the answers below are for different such types.
    – jcaron
    Commented Jul 31, 2019 at 13:44

6 Answers 6

59

Another way of looking at it is that they save cable runs, not ports. If you've already got a router and a long 100BaseT cable to a PC, and you want to add a second PC, you can install a splitter on each end of the cable, connecting to two router ports, and the two PCs.

As a diagram:

Old:

 _________     Cat5
|        1|===================[PC]
|        2|
| Router 3|
|        4|
|_________|

New:

 _________                 Cat5
|        1|---[splitter]=================[splitter]------[PC]
|        2|------/                           |
| Router 3|                                  \--------[New PC]
|        4|
|_________|

I've only ever seen these splitters sold in pairs, for precisely this reason. To use them singly requires customised cables at the other end. That's certainly possible, but if you've got the kit to do that, it's probably easier to do the job properly and run a second cable. If you're starting from scratch, run enough cables in the first place (in my house I run two cables if I think I need one).

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    As an IT Technician, this is how I have seen them used in the past. They would only work in pairs, one at each end of the ethernet cable.
    – TiO
    Commented Jul 29, 2019 at 9:03
  • 1
    "...if you've got the kit to do that, it's probably easier to do the job properly and run a second cable." I've seen these used mainly in situations where it's hard to run a second cable. E.g., there's a lack of space in the conduit, or you don't have easy access to the conduit to add another run.
    – cjs
    Commented Jul 30, 2019 at 1:48
  • @Curt that's reasonable. Maybe I went a little far with my note of caution but I wanted to discourage blind planning on splitters.
    – Chris H
    Commented Jul 30, 2019 at 6:52
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    re "if you've got the kit do it properly and run a second cable": hm. IMO i think "properly" would be[1] install a second router/hub at the location of the new pc because you get 1gbps which is better than 20-200mbps (and keep the router with one cable at 1gbps). ([1] to never use cat5. always use cat5e (which supports gigabit)). looks 88+ people agree "TL;DR. Buy a switch. Do not use Ethernet Splitters, EVER." and "Practically this might drop down to 10BaseT speeds... what you REALLY need is a switch". Commented Jul 30, 2019 at 15:06
  • 100BaseT only needs 2 of the 4 twisted pairs in the cable, so you can actually crimp two sets of RJ45's on each end, and plug them into two different ethernet ports. Done this with some walljacks going to phones that didn't need gigabit on the network.
    – SnakeDoc
    Commented Jul 30, 2019 at 18:25
55

Ethernet splitters are not mini-hubs.

They are specifically designed to split a single connection with four twisted pairs into two connections with two distinct twisted pairs.

Read this write up here for the basics:

100BASE-T requires only two twisted pairs. So basically half of a Cat 5e cable. Just four out of the eight available wires. So if we’re only using half of the wires in the cable, couldn’t we make one cable act as two cables? Why yes! Yes we can! And that’s exactly what an ethernet splitter does!

So basically an Ethernet Y-splitter will give you two 100Base-T or 10Base-T connections from a 1000Base-T connection. Most ethernet wiring setups have four twisted pairs even if they are not needed for most uses. Thus why Y-splitters exist.

I have tried splitting the cables that go into the router with a Y and it does not work.

Are you sure you are using a router and not a plain modem with a single Ethernet connection? If this is a plain modem with one Ethernet port then a Y-splitter will just confuse the connection. And if your ISP tracks MAC addresses, then adding a Y-splitter will only make your ISP think there is something wrong or broken with your network.

You are much better off hooking up a real router to your modem than using a Y-splitter.

0
8

When you find yourself in the situation where you run out of cable drops, you can install one of these splitters (also somethimes called "RJ45 Cable Economisers") between one port on your patch panel and two on your switch. You then install a second one at the single RJ45 wall outlet (coming from the single port on the patch panel) and then connect two devices to it.

What I think a lot of people misunderstand with these things is that they don't give you additional ethernet ports. They allow you to multiplex a pair of ethernet connections across a single cable drop.

You need two because you have to mux at one end and demux at the other. You plug two ethernet sources (e.g. ports on a switch) into one of these and connect it to a single cable run. You connect the other unit to the other end of that cable run and then connect two devices (e.g. computers) to it.

It works by using unused pairs in the cable run to run a second connection. A device like this is useful when it is impractical to run a new cable from the device to the router (for example, when the existing cabling is in a wall and/or a new cable run would need to penetrate walls and floors).

These devices are not suitable for Gigabit ethernet because that uses all 8 wires within the network cable, leaving none spare. They are suitable for use with 10 or 100 ethernet however because they only use 4 of the 8 wires.

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It's not a "splitter" per se, and shouldn't be marketed as one. It would be better to call it a "cable sharer".

From Wikipedia:

Shared cable

10BASE-T and 100BASE-TX require only two pairs (pins 1–2, 3–6) to operate. Since common category 5 cable has four pairs, it is possible to use the spare pairs (pins 4–5, 7–8) in 10- and 100-Mbit/s configurations for other purposes. The spare pairs may be used for power over Ethernet (PoE), for two plain old telephone service (POTS) lines, or for a second 10BASE-T or 100BASE-TX connection. In practice, great care must be taken to separate these pairs as 10/100-Mbit/s Ethernet equipment electrically terminate the unused pins.[citation needed] Shared cable is not an option for Gigabit Ethernet as 1000BASE-T requires all four pairs to operate.

Ethernet uses pins 1, 2, 3 and 6 because 4 and 5 would often be used for telephone services - a "6 pin, 2 connected" phone plug wired up to RJ11 fits into the RJ45 ethernet socket and makes contact with pins 4 and 5. By splitting ethernet up in this way one set of cabling in an office suite could support a shared use of the cable in the wall, which had 8 wires. Other ways of sharing and achieving multiple use of the wire were established too; some devices use the unused 4, 5, 7 and 8 for power, which means a single wire/plug/socket arrangement can both power a device and get data out of it.

Cable sharing devices like this Y device you have are another application of sharing the 8 core wire in the wall into two 4-wire plugs; you connect the single end of the Y into the wall, and the double end of the Y into two ports on your device (such as a router). At the other end you can connect two different ethernet devices into the double end of the Y and the single end of the Y again to the wall. After you have done this, your laptop is, for example, getting its internet over the orange pair and green pair set of wires inside the cable in the wall, and your desktop PC is getting its internet over the brown and blue pair. You have shared the use of the cable in the wall and made use of the formerly unused wires.

As other answers have pointed out, this is really only good for 100 megabits. If the cable in the wall supports gigabit and you want gigabit to both your laptop and desktop pc in the office, down to the router in the kitchen, then you buy a gigabit switch and put it in the office. You connect both computers to the switch then connect the switch, via the cable in the wall, to the router in the kitchen. Now everyone in the office has gigabit of some form. This is an electronic "sharing" of the ethernet network between the two computers in the office, versus a Y sharer, which achieves a physical sharing. The switch will always be the better route because of the number of devices it can support

1

Another use I've seen for passive Ethernet splitters is to use a network sniffer when you otherwise can't use a switch's mirror port (which exactly copies traffic from one or more other switch ports) and you don't have a hub (or similar) handy.

You put it inline between two devices and connect to a third device, where you're running Wireshark or another tool to capture the traffic.

If you're not running gigabit, this method can only see half of the traffic. Why half? Because the network card can only physically receive on one pair at once (for fast ethernet) and there are two pairs, one each for TX and RX in the source.

For example, Great Scott Gadgets makes the Throwing Star LAN Tap which allows you to choose which pairs you're connected to.

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    This is unlikely to apply to whatever commercially-available splitter the OP has obtained. Commented Jul 29, 2019 at 18:14
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    Gigabit Ethernet doesn't have RX/TX pairs, it can and does physically transmit and receive on all four pairs simultaneously. It uses echo cancellation to achieve this (the card subtracts what data it has just sent to determine what it received). Commented Jul 29, 2019 at 18:32
  • @grawity: You're right, I'd half edited my reply but hadn't completely finished it. I'll fix it. Commented Jul 30, 2019 at 19:53
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I looked up "Ethernet splitters" on Amazon, and the various descriptions state:

  • NOTE: Don't support 2 computers that connect to the internet simultaneously, ONLY 1 output port is working.

    This RJ45 Ethernet Splitter is not a switch so that it can not implement 2 computers sharing the internet at the same time.

    Application: When you often use a laptop in 2 different fixed position, using this RJ45 splitter, you don't have to worry about no Internet in your rooms.

  • This Adapter Advantage Design: A notebook in two different fixed position often back and forth use, this is a very trouble, the network cable to lead to broken, this rj45 1 to 2 ways RJ45 ethernet splitter help you do not have to pull back and forth cable. NOTE: This Ethernet splitter allows two computers to share one Ethernet line ONE AT A TIME, but it doesn't support both computer to connect onto the internet simultaneously

    Expand one RJ 45 outlet into two, standard 8P8C design. Please noted that only One port could be used at one time

  • ONLY ONE Female Port Workable At A time

    NOTE: This Ethernet splitter allows two computers to share one Ethernet line ONE AT A TIME, but it doesn't support both computer to connect onto the internet simultaneously

  • RJ45 Ethernet Network Splitter could help to split one network cable to 2 port output, that's convenient to take turns to use different computers. (Please note that cannot support using two cables at the same time)

You get the gist...

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  • This sounds like a different kind of splitter - not made for making two 4-wire connections on one 8-wire cable, but simply a ghetto bridge tap. Commented Jul 30, 2019 at 19:05
  • @rackandboneman it is indeed a different type of splitter from the ones most answers refer to, but the OP’s question does not say what type of splitter is used, and that’s the kind of splitter you get when you search on Amazon and probably many other sites.
    – jcaron
    Commented Jul 30, 2019 at 22:00
  • Not sure why the downvotes... I'm describing the type of splitter OP had a highest probability of having bought, and the explicit warnings and use cases given in their descriptions, which answers both questions: why they sell them (use case) and why it doesn't work (warning)...
    – jcaron
    Commented Jul 31, 2019 at 13:42
  • Interesting side fact: 10base2 and 10base5 ethernet are actually designs that do use a bridge tapped connection... not on TP cable though. Commented Aug 1, 2019 at 21:35
  • This shouldn't have been downvoted. There is a troll problem here at SU. Commented Oct 22, 2020 at 19:44

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