Network Address Translation (NAT) is technically what Cisco refers to as translating one IP address to another. The majority of installations, including most home networks, translate many IP addresses to a single address. This is actually called Port Address Translation (PAT). PAT has also been called NAT Overload in IOS.
To complicate matters, in the PIX OS, NAT is used in a number of ways that may not
seem obvious. For example, you may have to use a
statement to allow packets from one interface to another, even though they both have
public IP addresses, and would normally require no translation.
A few commands are used to configure the majority of NAT scenarios. Some, such as
nat command, have many options that I will not
list here. The subject of NAT on a PIX firewall could fill a book itself. My goal is to
keep it simple. If you need more information than what I've provided here, the Cisco
command references are a good place to start. The commands you're most likely to need
nat command is used when translating
addresses from a more secure interface to a less secure interface. For example, if
you needed to translate an address on the inside of your PIX to an address on the
outside, you would use the
nat command. Private
IP addresses on the inside of a PIX are translated to one or more public IP
addresses using the
nat command. (Technically, the addresses do not need to be private and public addresses as described by RFC1918. The PIX documentation ...