Linux Firewalls, Third Edition

iptables NAT Semantics

iptables provides full NAT functionality, including both source (SNAT) and destination (DNAT) address mapping. The term full NAT isn't a formal term; I'm referring to the capability to perform both source and destination NAT, to specify one or a range of translation addres-ses, to perform port translation, and to perform port remapping. iptables supports the three general types of NAT (traditional NAT, bidirectional NAT, and twice NAT), as defined in RFC 2663.

A partial implementation of NAPT, known as masquerading among Linux users, was provided in earlier Linux releases. It was used to map all local, private addresses to the single public IP address of the site's single public network interface.

NAT and forwarding were ...