RJ45 doubler that allows 2 computers on one plug

Solution 1:

This is called passive ethernet hub. The schematic is quite simple, but only works for three devices (two computers and one upstream switch):

(EEWeb: Building a Passive Ethernet Hub)

In practice these are quite fiddly even at 10 Mbps, and have been superseded by either powered switches or those pair splitters that you put at both ends and that work up to 100 Mbps.

Solution 2:

I have a 24 port old switch, but it uses 18 watts even when there is no traffic

Newer devices tend to consume less power than older ones.

I measured consumption of my D-Link 8-port Gigabit switch DGS-108. It consumes about 1W with a few devices connected.

With disconnected devices it consumes slightly less.

I also measured similar 5-port version DGS-105, and it consumed only slightly less than DGS-108.

One more thing to think about is opportunity cost (alternative cost). I like the idea to see our time as life currency. If you want you can spend hours choosing most efficient switch or one with least CO2 impact. Or you can spend hours planting trees. Or you can spends hours working more and then donating extra earned money to foundation which for example fights with climate change.

But you can't do all things, because a day has only 24h. Which means that by doing something you won't do something else in the same time.

I don't know which way is best, but I think after some experience you will notice that some ways to help planet may be more effective than others. And you may or may not come to conclusion, that it may be better to choose any switch now and spend this time doing something with more impact.

Solution 3:

I have had an encounter with such a setup--although hardwired--some years ago. (Don't let electricians near a network wire!!) Despite low loads from the two computers involved it caused noticeable network lag.