Linux.com: Netfilter for IP Masquerade

“As of 2.4, ipchains is a thing of the past. The replacement for
ipchains is Netfilter’s iptables. What does this mean to the end
user? Typically it means little beyond the fact that suddenly their
ipmasq script doesn’t work. So, for starters let’s get into setting
up ipmasq under 2.4.x kernels.”

• “First, check that you’ve got network packet filtering enabled,
  you’ll find it under the networking options menu.”
• “Enabling packet filtering will add a submenu lower down called
  Netfilter configuration.”
• “In the Netfilter’s configuration menu be sure to enable
  “Connection tracking”, “IP Tables support” and “Full NAT”
  support.”

“Connection tracking is a new feature of netfilter that allows
you to accept or deny a packet based on the state of the connection
rather than the strict allow/deny of ipchains. NAT of course stands
for Network Address Translation, the key feature behind IP
Masquerade. The other entries in this menu are optional, enable
them as you please and recompile the kernel if needed.”

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