Network cable splitter not working

Question

I'm working on a home set-up where I can split my network cable to use two computers on one side, and two ports on my modem on the other side (so a 2-1-2 config). I have the following set-up:

• 1 (longer) gigabit network cable between the 2 computers and my modem.
• 2 ISDN RJ45 splitters
• cat 5 cables between my two computers and one splitter, and two cat 5 cables between the other splitter and two ports on the modem.

However, whenever I have both cables connected on my modem and just one cable (to one PC) to the splitter, my PC cannot get a connection (it just shows unplugged). Whenever I take out one of the cables on the modem side, it starts working again.

I've understood this should work as long as I also have it connected to two ports on the modem, but I can't get it to work. Do I need different cables? Or is this a method that just doesn't work anymore?

I understand this is also not a preferred method (and something like a switch would be better), but I am trying to run this cheaply for a short period of time, and running two long cables is not an option unfortunately.

Solution

It sounds as if you have a basic RJ45 splitter and not an Ethernet splitter, and that your splitter is connecting all four cable pairs to both terminals*.: You are likely to have at least two types of problem: reduced signal levels due to the additional loading (whether this is a problem will depend on how the equalisation is managed), and reflections of your transmitted data from the joint. There could be some double reflections, depending on how well the IOs of the terminals match the cable.

Even (basic) 1Gb/s Ethernet switches are really good value these days, but you probably only need 100-Mb/s, and there are reliable switches on the market at really low.
If you don't have enough local power sockets or don't want to clutter your floor you can use an Ethernet splitter, but a reliable splitter won't be any cheaper than a 100Mb/s switch. Ethernet splitters connect two of the four pairs in the cable to each terminal, which avoids both reflections and attenuation issues.

Comments

[headkase]

I believe you need a minimum of a hub and better a switch - which you've said you don't want. A switch is better than a hub. Also, the modem must be able to assign more than one IP address which means it'll need router functions or you'll need to talk to your ISP to get an extra IP.

[headkase]

And if you have a cheap router lying around just run one cable from the modem and use the router to split it and also assign IPs. That would be a good method if you do have one kicking around.

[Deniz Zoeteman]

@headkase Sorry, I should have specified, it's a router/modem combo from my ISP.

[Deniz Zoeteman]

The splitter seems to have the same connections on both ports, and I can connect my PC to either port, and it works fine. All info I can find on the splitter is that it's RJ45 female on all ports, and labelled as "8p8c" on the one side and 2x "8p4c" on the other side (my networking knowhow isn't great, so I don't know what that means, but maybe you know more). I know a switch isn't super expensive, but it's also another device I have to keep powered and have around, while I won't have this set-up for long. The splitters were only 2 EUR each ;)