"Theoretically," you are NOT allowed to connect more than one computer to any of those brandband services unless otherwise specified. But the fact is that you can accomplish that easily with a hub/switch/router.
Hub: cheap, but as described by MrDJay, heavy network communication overehead
Switch: intermediate, most collision problems won't be observable unless multiple machines tries to pull something from the Internet simultaneously (you'll be limited by the broadband downlink speed anyway...)
Router: relatively ex as compared to the other two, but you can get rid of the "server" as router can act on your behave.
If you do wanna use a server + hub/switch, a simple gateway software running on the server will do the magic.
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