Cisco Systems ASA 5585-X Manual De Usuario

Twice NAT lets you identify both the source and destination address in a single rule. This chapter shows 
you how to configure twice NAT and includes the following sections:

For detailed information about how NAT works, see 

Twice NAT lets you identify both the source and destination address in a single rule. Specifying both the 
source and destination addresses lets you specify that a source address should be translated to A when 
going to destination X, but be translated to B when going to destination Y, for example.

For static NAT, the rule is bidirectional, so be aware that “source” and “destination” are used in 
commands and descriptions throughout this guide even though a given connection might originate at the 
“destination” address. For example, if you configure static NAT with port address translation, and 
specify the source address as a Telnet server, and you want all traffic going to that Telnet server to have 
the port translated from 2323 to 23, then in the command, you must specify the source ports to be 
translated (real: 23, mapped: 2323). You specify the source ports because you specified the Telnet server 
address as the source address.

The destination address is optional. If you specify the destination address, you can either map it to itself 
(identity NAT), or you can map it to a different address. The destination mapping is always a static 
mapping.