iptables and netfilter are building blocks of a framework inside the Linux 2.4.x and 2.6.x kernel. This framework enables packet filtering, network addresss [and port] translation (NA[P]T) and other packet mangling. It is the re-designed and heavily improved successor of the previous Linux 2.2.x ipchains and Linux 2.0.x ipfwadm systems.

iptables is a generic table structure for the definition of rulesets. Each rule within an IP table consists out of a number of classifiers (iptables matches) and one connected action (iptables target).

netfilter, iptables and the connection tracking as well as the NAT subsystem together build the whole framework.

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Here are some key features of "iptables":

· stateless packet filtering (IPv4 and IPv6)
· stateful packet filtering (IPv4)
· all kinds of network address and port translation (NAT/NAPT)
· flexible and extensible infrastructure
· multiple layers of API's for 3rd party extensions
· large number of plugins/modules kept in 'patch-o-matic' repository