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 ASA, NAT is used in a number of ways
that may not seem obvious. For example, you may have to use a
nat 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 the
nat command, have many
options that aren’t listed here. The subject of NAT on an ASA 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
is used when you’re translating addresses from a
more secure interface to a less secure one. For example, if you
need to translate an address on the inside of your ASA to an
address on the outside, use the
nat command. Private IP addresses on the
inside of an ASA are translated to one or more public IP addresses
nat command. Technically, the addresses do not need to be private and public addresses, as described by RFC1918. The ASA documentation uses the terms “global” ...