Will an Ethernet splitter work?

TL;DR. Buy a switch. Do not use Ethernet Splitters, EVER.

Midlan posted a link to this schematic:

'Parallel wired' means that if you use a device like that at best you will get a load of packet collisions because you have wired the two computers (Jacks 1&2) TX pins (Pair 3, Pins 1&2) together, and the RX (Pair 2, Pins 3&6) pins together.Ethernet Wiring

Twisted pair Ethernet, 10base-T, 100base-TX, 1000bast-T, etc. all need to be connected end-to-end. At each there is a transmission (TX) pair, and reception (RX) pair. This is how a cross-over cable works.

Indeed the simplest Ethernet network using twisted pair media is using a crossover cable between two computers:

 -----------------------------------------
 |           Computer-A                  |
 |           568A                        |      
 | Pair 3 - Pin 1 - TX+ Green on White   +-------\
 | Pair 3 - Pin 2 - TX- Green            +=======|==\
 | Pair 2 - Pin 3 - RX+ Orange on White  +-\     |  |
 | Pair 1 - Pin 4 - B+  Blue             + |     |  |
 | Pair 1 - Pin 5 - B-  Blue on White    + |     |  |
 | Pair 2 - Pin 6 - RX- Orange           +=|==\  |  |
 | Pair 4 - Pin 7 - B+  Brown on White   + |  |  |  |
 | Pair 4 - Pin 8 - B-  Brown            + |  |  |  | 
 |                                       | |  |  |  | 
 ----------------------------------------- |  |  |  |
                                           |  |  |  |
 ----------------------------------------- |  |  |  |
 |          Computer-B                   | |  |  |  | 
 |          568B                         | |  |  |  |     
 | Pair 2 - Pin 1 - TX+ Orange on White  +-/  |  |  |
 | Pair 2 - Pin 2 - TX- Orange           +----/  |  |
 | Pair 3 - Pin 3 - RX+ Green on White   +-------/  |
 | Pair 1 - Pin 4 - B+  Blue             +          |
 | Pair 1 - Pin 5 - B-  Blue on White    +          |
 | Pair 3 - Pin 6 - RX- Green            +==========/
 | Pair 4 - Pin 7 - B+  Brown on White   +
 | Pair 4 - Pin 8 - B-  Brown            +
 |                                       |
 -----------------------------------------

As you can see, the TX pins on Computer-A are wired to the RX pins on Computer-B, and similarly, the RX pins on Computer-A are wired to the TX pins on Computer-B. (For simplicity's sake, I have not wired up pins 4,5,7 & 8, but for completeness they should be wired straight through pin 4 to 4, 5 to 5, etc.)

What your Ethernet Splitter is doing is just adding in a Computer-C beside Computer-B, so that Computer B&C's pins are wired together, pin 1 to pin 1, 2 to 2, 3 to 3 etc. At best, your devices will not work, at worst you will damage your Ethernet ports.

Computer-A could infact be a hub or a switch, but you still have the problem of Computer-B's and Computer-C's TX and RX ports being wired together.

Here is a wiring diagram for a simple (passive/unpowered) Ethernet hub: http://www.eeweb.com/blog/circuit_projects/building-a-passive-ethernet-hub

https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair

Ethernet is a digital signal, and it is not like an analogue telephone signal where you can use a splitter to add in another extension. Each little wave pattern is a packet of information that is transmitted from a TX port that is intended to go a RX port. Wiring TX ports together is going to cause all sorts of weirdness.

Instead of a splitter, your best option is to add a mini-switch, but you need to be careful with your wiring topology if you already have multiple other switches in your network.

There were other search results which mapped the two unused pairs (1&4) on 100 Base-TX to pins on 1 2 3 and 6 on the second port, so you would have to use these device on each end. However, the Ethernet wire protocol has been designed to use twisted pairs in such a way that cross-talk is eliminated between the wires. Start doing non-standard, non-compliant things, and you will end up getting non-standard, non-compliant, unexpected results.

It's useful to understand what a splitter does. It turns one 8-strand ethernet cable into a what would be, essentially, a pair of sub-standard 4-strand cables that in theory should do Fast Ethernet (100BaseT/TX). Practically this might drop down to 10BaseT speeds, and you need to use a splitter on both ends for it to work. It will take up two ethernet ports on the far end. It will absolutely not work for Gigabit (1000Base) Ethernet, as that requires all 8 strands.

A hub and a switch are very different things.

A hub takes in data and retransmits it to all the ports (and I've never seen a fast Ethernet hub, let alone gigabit). A switch takes in data and switches packets only where they need to go (which is faster and more secure). This... well is a fairly dumb device that turns one cable into 'two'.

In theory it should work like an Ethernet hub, which only amplifies signal, but send it to all clients (like a splitter would). I've never used it, so I need confirmation that it work or not before I'll buy it.

Your theory is wrong, and what you REALLY need is a switch.