Netfilter is a utility in Linux 2.4 (and later releases) that facilitates Network Address Translation (NAT), packet filtering, and packet mangling. NAT is the process of converting an Internet protocol (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.
Together, the netfilter and the iptables commands constitute the successor to ipchains and ipfwadm in earlier versions of Linux. Netfilter and iptables are often combined into the single expression netfilter/iptables as a way to refer to the Linux subsystem for Network Address Translation, firewalling, and advanced packet processing.