Charter will only allow one Mac address at a time connected to the cable modem. They want you to pay extra for other "PCs" to share the connection. You have two choices:
1. Unplug power to the cable modem, clearing your PCs MAC address from the cable modem's NVRAM. Then plug in the Amiga and power up the modem again. You have to do this each time you want to switch computers. Needless to say, this solution sucks. The other option is to do what I did and...
2. Invest $54.99 in a Leviton cable modem router. Dlink also makes one, but the Leviton is much better because it works right out of the box and requires no software. The Dlink will work, but you have to configure it manually through it little embedded web server to make the Amiga work with it. The address of the Dlink's config page is 192.168.1.1. The leviton router can be found at big hardware stores, usually.. (Home Depot, etc.). The Dlink is about $45 at Radio Shack. Of the two, I strongly recommend the Leviton.