how to get a public ip from cable modem on a guest

[j2sc]

host; ubuntu 14.04 headless
virtualbox 4.3.28
guest; debian distro

Is there a setup for a virtualbox guest to get a public ip from a cable modem?

Currently the guest can get an ip by dhcp from a router, but it cannot get an ip by dhcp from a cable modem.

[Perryg]

AFAIK it is not possible. Your cable modem has one address period and that can not be shared with another PC, which is what you actually have.

[j2sc]

Shoot, I've tried a lot of different configs and even with the host adapter set to unassigned the modem must still see the host mac and just won't accept the guest mac. grrrrr.
That means I have to set up the host as a simple router but I'll need forwarding. sigh.

[j2sc]

And so, because of @Perryg's response I worked out another, complicated solution using vbox nat and natpf that worked fine.

But, after spending all that time doing that something occurred to me and after reconfiguring the ipmi and reconfiguring vbox I was able to have the guest obtain an external ip directly from the cable modem.

That only cost me a few hours. sigh.