If you haven't configured the ethernet interface to your internal network yet, now's the time to do it. The easiest way to do this is to run the control-panel program under X Windows to create a new interface, eth1. Select "none" as the protocol interface. Define the IP address. In the Routing dialogue, be sure to enable IP Forwarding (missing this detail causes people a lot of problems). Activate the interface.
Several ranges of IP addresses have been defined for private, internal, intranet use. The range for a C Class network is from 192.168.0.0 to 192.168.255.255. The third quad represents the possible network interfaces. The fourth quad represents hosts on that subnetwork.
By convention, 192.168.n.1 is usually the interface machine to the external network. 192.168.n.0 and 192.168.n.255 are reserved addresses. Don't use them.
The examples in this document use 192.168.1.1 to identify the Linux interface to the internal network. 192.168.1.2 identifies the second computer on the internal network.
If your second ethernet card was recognized when you rebooted last time, the internal network should be available now. If not, reboot.
14.1 Windows Setup
I don't have a Windows machine. Until this section is filled in by someone else, if you need setup information, refer to IP Masquerade mini-HOWTO.
14.2 Macintosh Setup
Go to your TCP/IP Control Panel. Create a new configuration by using the Configurations dialogue found under the File Menu. Use the following settings:
Connect via: Ethernet
Configure: Manually
IP Address: <Macintosh's IP address>
Subnet mask: <your mask>
Router address: <Linux internal interface address>
Name server addr: <list of name server IP addresses>
Starting domain name: <your domain name>
Note: The name server addresses are taken from /etc/resolv.conf/. They can change at any time, although they probably won't.