Iptables
iptables is an application used to configure tables provided by the Linux kernel firewall. The application is run by system administrators and must be run with elevated privileges. It must also be executed by user root or it will not function. Typically the iptables application is installed in /usr/sbin/iptables, but may also be installed in /sbin/iptables. Documentation on the installation can be found in the man page, opened by executing the command "man iptables".
Before iptables, to create Linux firewalls, users needed to use ipfwadm in Linux 2.0 and ipchains in Linux 2.2. iptables continued with some of the basic ideas implemented in ipfwadm. It also expanded upon the idea of chains of rules, which was implemented in ipchains, by implementing tables.
Also see: IP, Network definitions