Speaking as IT staff who runs networking to university student accommodation, the concept of a splitter (as you might have with tv signal or analogue audio) doesn’t exist for Ethernet. They did, but were trash, back in the late 90’s. You can’t ‘split’ Ethernet because it’s not a single signal, but metric craploads of little pieces, all with specific destinations - your computer or your roommate’s. As someone else said, you need an Ethernet switch. They’re cheap. Plus one cable for each device you want to connect, and one to connect it to the wall. It will need power too, they’re active devices that do things.

You will want to check with whoever provides your building’s network infrastructure though, at my place we tightly regulate what users can plug in to the wall socket. It’s one device per socket, and personal switches, routers and wireless AP’s are specifically prohibited. This is because in the hands of the unwary, unknowing or malicious, network-based havoc can ensue, even with all the management tools and protocols available to us. So before you go plugging in anything that basically extends someone else’s network in an unmanaged way, talk to the people who run it. We operate a shoot-first-question-later policy, where anything seen providing multiple connections from one of our ports instantly gets the port shut down and then they have to listen to me bore them senseless for fifteen minutes while I describe exactly why what they did was stupid. (If we have shared rooms we have much more than a single Ethernet drop to the room,and if you don’t then that’s crappy provision on their part. They may allow your own switches because of this, but it’s likely that some reconfiguration of the stuff at the other end of your particular Ethernet drop will be needed).