What device can you use to connect two or more computers with a single cable modem?

You can use a router, either wired (Ethernet) or wireless to connect two or more computers with a single cable modem. You would need either Category 5, 5e, or Category 6 Ethernet cables to do this. Also, you may set up a wireless Local Area Network (otherwise known as LAN) if your type of router supports that and your computer(s)/devices support it.