Which IPs for Which Devices?

[Art Lemasters <alemas@rmi.net>]

     I have one Linux box (with DNS, etc.) connected to the Internet with
a modem (PPP).  I want to connect it with a router (routing PPP) via one 
ethernet card (eth0).  Should all three devices (Linux system, ethercard
and router) have the same IP address...two have private addresses and one
public?  ...which ones should have which IPs?  All of the documentation
I've found so far only addresses nets with a whole range of IP addresses,
but I only have _one static IP_.     

Art