Building a Linux Network Appliance, Part 2 - page 3
What to Build
You only need to configure a single network interface card (NIC), because for now all you need is Internet connectivity.
If your future gateway box is on your LAN, and not directly connected to the Internet, go ahead and manually configure a static IP for it, since you'll need one anyway. For example, since my subnet is 192.168.1.0/24, I configured mine with an IP of 192.168.1.15, netmask 255.255.255.0, gateway 192.168.1.10 (which is the LAN address of my own Internet gateway), and I used the nameserver of my ISP.
If your future gateway box is connected directly to the Internet, use the account information supplied by your ISP.