Router Linux

[VIALIDAD]

The reason for the present is to consult on the configuration of ruter
linux. 
The problem that I have is that not the IP that I must assign to the
network
interface eth0 (network public or the one that is connected with modem
Cisco
677).  The other interface (eth1) this connected to the private network
and
assigns IP to him 192.168.0.11 mask 255.255.255.0 the IP that assigns
the
ISP is dynamic and the IP of modem Cisco is 10.0.0.1 mask 255.255.255.0.

Concretely as it is the IP I must assign to eth0 of router linux?

Marcelo.