The system is typically connected to the network by one network card only. Both public, and private IP address may be used. Should you, however, wish to separate access from the Internet and from the private network, you can connect the system via 2 network cards. On the first one, there will be the public IP (the access will be allowed from selected number of IP addresses on the firewall due to security), and the second one will be connected into the private network (e.g. client portal).

Do the configuration from linux console by editing /etc/rc.local:

ifconfig eth1 192.168.168.100 netmask 255.255.255.0 route add default gw 192.168.168.1
ifconfig eth0 8.8.8.8 netmask 255.255.255.0 route add default gw 8.8.8.1