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Cisco ASA 5500 Series Configuration Guide using ASDM
OL-20339-01
Chapter 28 Configuring Twice NAT
Configuring Twice NAT
You can also create a new named object or group from the Browse Original Destination Address
dialog box and use this object or group as the real destination address.
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 26-15.
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 from the Browse Original Service dialog box.
You can also create a new service object from the Browse Original Service dialog box and use this object
as the real port.
A service object can contain both a source and destination port. You should specify either the source or
the destination port for both the real and mapped 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.
Step 5 Choose Static from the Match Criteria: Translated Packet > Source NAT Type drop-down list. Static is
the default setting.
This setting only applies to the source address; the destination translation is always static.
Figure 28-20 Setting the NAT Type
Step 6
Identify the translated packet addresses (the mapped source address and the real destination address).
a. For the Match Criteria: Translated Packet > Source Address, click the browse button and choose
an existing network object or group from the Browse Translated Source Address dialog box.
Figure 28-21 Browse Dialog Box
You can also create a new named object or group from the Browse Translated Source Address dialog
box and use this object or group as the mapped source address.
For static NAT, the mapping is typically one-to-one, so the real addresses have the same quantity as
the mapped addresses. You can, however, have different quantities if desired.
For static interface NAT with port translation, you can specify the interface instead of a network
object/group for the mapped address. For more information, see the “Static Interface NAT with Port
Translation” section on page 26-5.
See the “Guidelines and Limitations” section on page 28-2 for information about disallowed
mapped IP addresses.
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