You need to put a router between your machines and the cable modem. This will receive the single IP you are allowed and do Network Address Translation (NAT) to allow any number of other machines connected to it to access the internet. A bridge or hub will not work because they aren't smart enough to deal with the single IP requirement.
"Similar to Chris Nava's answer I also believe you need some kind of NAT. But maybe that's possible without plugging an extra piece of hardware (router) into your network: I found an article describing how to enable NAT for a container with private IP addresses. I hope that helps."
