cable modem connected to ethernet switch.

I have an Arris TM822G cable modem that connects to Videotron Internet; not sure of the specifics. When I connect a PC directly to it, the PC receives a public IP address on a /24 subnet. When I attempt a 2 client network, only one PC will work at a time. Although they both receive different IPs, they just won't play nice together. Question; does Videotron limit me to just one public IP address on the /24 subnet? ... or is my switch the issue? Thankyou in advance.