iptables is Linux's firewall which has been a part of the kernel since version 2.4. It is often referred to as a packet filter as it examines each packet transferred in every network connection to, from and within your computer. iptables replaced ipchains in the 2.4 kernel and added many new features including connection tracking (also known as stateful packet filtering). In this article we will use iptables to build simple but effective firewalls for the following scenarios using allow/disallow rules based on IP addresses, ports and states:

* a standard home computer;
* a home/small office network with a single internet connection;
* port forwarding for a home/small office network.