Buy a router, which will have 4 LAN ports and an 8 port switch. Connect the router WAN port to your Internet and attach the one of the switch ports to an LAN port on the router. Then attach all the computers to either an open LAN port on the router or one of the switch ports. Then just configure the router to assign DHCP addresses and give it a large enough range to cover all devices.
Best to get a gigabit router and switch, as they are inexpensive and will allow any computers attached than have gigabit LAN to transfer files among them much faster. All computers will share the 1mbps bandwidth and most good routers will allow you to prioritize users/uses.
You could also use the wireless feature of the router if any of the machines have wireless cards.