Iptables is a generic table structure that defines rules and commands as part of the netfilter framework that facilitates Network Address Translation (NAT), packet filtering, and packet mangling in the Linux 2.4 and later operating systems. NAT is the process of converting an Internet Protocol address (IP address) into another IP address. Packet filtering is the process of passing or blocking packets at a network interface based on source and destination addresses, ports, or protocols. Packet mangling is the ability to alter or modify packets before and/or after routing.
Iptables and netfilter are the successor to ipchains and ipfwadm in earlier versions of Linux. Netfilter and iptables are often combined into the single expression netfilter/iptables, which refers to the Linux 2.4 and later subsystems for NAT, firewalling, and advanced packet processing.