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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 34 Configuring Twice NAT (ASA 8.3 and Later)
Configuring Twice NAT
Step 3 Identify the original packet addresses; namely, the packet addresses as they appear on the source
interface network (the real source address and the mapped destination address). See the following figure
for an example of the original packet vs. the translated packet where you perform identity NAT on the
inside host but translate the outside host.
a. For the Match Criteria: Original Packet > Source Address, click the browse button and choose an
existing network object or group or create a new object or group from the Browse Original Source
Address dialog box. The default is any; only use this option when also setting the mapped address
to any.
b. (Optional) For the Match Criteria: Original Packet > Destination Address, click the browse button
and choose an existing network object or group or create a new object or group from the Browse
Original Destination Address dialog box.
Although the main feature of twice NAT is the inclusion of the destination IP address, the destination
address is optional. If you do specify the destination address, you can configure static translation for
that address or just use identity NAT for it. You might want to configure twice NAT without a
destination address to take advantage of some of the other qualities of twice NAT, including the use
of network object groups for real addresses, or manually ordering of rules. For more information,
see the “Main Differences Between Network Object NAT and Twice NAT” section on page 32-16.
Step 4 (Optional) Identify the original packet source or destination port (the real source port or the mapped
destination port). For the Match Criteria: Original Packet > Service, click the browse button and
choose an existing TCP or UDP service object or create a new object from the Browse Original Service
dialog box.
A service object can contain both a source and destination port. You should specify either the source or
the destination port for both service objects. You should only specify both the source and destination
ports if your application uses a fixed source port (such as some DNS servers); but fixed source ports are
rare. In the rare case where you specify both the source and destination ports in the object, the original
packet service object contains the real source port/mapped destination port; the translated packet service
object contains the mapped source port/real destination port. NAT only supports TCP or UDP. When
translating a port, be sure the protocols in the real and mapped service objects are identical (both TCP
or both UDP). For identity NAT, you can use the same service object for both the real and mapped ports.
The “not equal” (!=) operator is not supported.
Real: 192.168.1.1
Mapped: 10.1.1.1
10.1.2.2
Identity
NAT
Outside
Inside
10.1.2.2 ---> 10.1.1.1 10.1.2.2 ---> 192.168.1.1
Original Packet Translated Packet
Source Destination