“Now that you’ve configured and compiled the Linux kernel to
support IP Masquerading, we’re going to setup your Ethernet cards,
and configure the kernel firewall.“
“In the first part we talked about “private” IP addresses. In
the following example, we’ll be using addresses in the range from
192.168.1.1 to 192.168.1.254. This is part of one of three special
blocks of addresses set aside for the purpose of building a private
network. These addresses are not routable on the Internet, and if
such a network address were to be used to communicate directly with
the Internet, packets coming from that address would be discarded
by the first router they encounter. For more on private IP
addresses, please see the document RFC 1918.”
“First, we’ll deal with the case in which you have two Ethernet
cards: one connected to your cable/DSL modem, which we’ll call
eth0; and another connected to an Ethernet hub, which we’ll call
eth1. Since most Internet service providers hand out IP addresses
dynamically whenever you boot your computer, configure eth0 to
obtain an IP addresses dynamically (typically using the Dynamic
Host Configuration Protocol). Please see the DHCP mini-HOWTO for
distribution-specific information on how to handle this.”