Well... many DSL and cable modems in the USA are only
equipped to hand
out DHCP to only one device, and will not talk to devices plugged-in
after that. If it has only one network port, it probably will only talk
to one device. If it has four ports, it is no doubt a modem plus a
firewall/router, and will support four devices.
I suppose since I haven't met anywhere near all the DSL and cable
providers in the US, that I could be wrong, but... this sounds like a
grievous misunderstanding of the common "one external IP address"
service. The device will hand you more than one internal IP. If it's
equipped with one port and not doing NAT, then it is expecting you to
supply a firewall device, not to plug directly into it. The firewall
device will then do NAT and hand you multiple internal addresses.
De