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Step 11Set the real and mapped interfaces:
Step 12For the Original Source Address, type the name of the inside network object (myInsideNetwork) or click 
the browse button to choose it.
Step 13For the Original Destination Address, type the name of the Telnet/web server network object 
(TelnetWebServer) or click the browse button to choose it.
Step 14For the Original Service, click the browse button to add a new service object for HTTP in the Browse 
Original Service dialog box.
a.Add the new service object.
b.Define the protocol and port, and click OK. 
						

							 
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c.Choose the new service object by double-clicking it. Click OK to return to the NAT configuration.
Step 15Set the NAT Type to Dynamic PAT (Hide):
Step 16For the Translated Source Address, click the browse button to add a new network object for the PAT 
address in the Browse Translated Source Address dialog box.
a.Add the new network object.
b.Define the PAT address, and click OK. 
						

							 
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c.Choose the new network object by double-clicking it. Click OK to return to the NAT configuration.
Step 17For the Translated Destination Address, type the name of the Original Destination Address 
(TelnetWebServer) or click the browse button to choose it.
Because you do not want to translate the destination address, you need to configure identity NAT for it 
by specifying the same address for the Original and Translated destination addresses.
Step 18Click OK to add the rule to the NAT table.
Step 19Click Apply. 
						

							 
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Feature History for Twice NAT
Ta b l e 5 - 1 lists each feature change and the platform release in which it was implemented. ASDM is 
backwards-compatible with multiple platform releases, so the specific ASDM release in which support 
was added is not listed.
Table 5-1 Feature History for Twice NAT
Feature NamePlatform 
Releases Feature Information
Twice  NAT 8.3(1) Twice NAT lets you identify both the source and destination 
address in a single rule.
We modified the following screen: Configuration > Firewall 
> NAT Rules.
Identity NAT configurable proxy ARP and route 
lookup8.4(2)/8.5(1) In earlier releases for identity NAT, proxy ARP was 
disabled, and a route lookup was always used to determine 
the egress interface. You could not configure these settings. 
In 8.4(2) and later, the default behavior for identity NAT 
was changed to match the behavior of other static NAT 
configurations: proxy ARP is enabled, and the NAT 
configuration determines the egress interface (if specified) 
by default. You can leave these settings as is, or you can 
enable or disable them discretely. Note that you can now 
also disable proxy ARP for regular static NAT.
For pre-8.3 configurations, the migration of NAT exempt 
rules (the nat 0 access-list command) to 8.4(2) and later 
now includes the following keywords to disable proxy ARP 
and to use a route lookup: no-proxy-arp and route-lookup. 
The unidirectional keyword that was used for migrating to 
8.3(2) and 8.4(1) is no longer used for migration. When 
upgrading to 8.4(2) from 8.3(1), 8.3(2), and 8.4(1), all 
identity NAT configurations will now include the 
no-proxy-arp and route-lookup keywords, to maintain 
existing functionality. The unidirectional keyword is 
removed.
We modified the following screen: Configuration > Firewall 
> NAT Rules > Add/Edit NAT Rule
PAT pool and round robin address assignment 8.4(2)/8.5(1) You can now specify a pool of PAT addresses instead of a 
single address. You can also optionally enable round-robin 
assignment of PAT addresses instead of first using all ports 
on a PAT address before using the next address in the pool. 
These features help prevent a large number of connections 
from a single PAT address from appearing to be part of a 
DoS attack and makes configuration of large numbers of 
PAT addresses easy.
We modified the following screens: Configuration > 
Firewall > NAT Rules > Add/Edit NAT Rule. 
						

							 
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Round robin PAT pool allocation uses the same 
IP address for existing hosts8.4(3) When using a PAT pool with round robin allocation, if a host 
has an existing connection, then subsequent connections 
from that host will use the same PAT IP address if ports are 
available.
We did not modify any screens.
This feature is not available in 8.5(1) or 8.6(1).
Flat range of PAT ports for a PAT pool 8.4(3) If available, the real source port number is used for the 
mapped port. However, if the real port is not available, by 
default the mapped ports are chosen from the same range of 
ports as the real port number: 0 to 511, 512 to 1023, and 
1024 to 65535. Therefore, ports below 1024 have only a 
small PAT pool.
If you have a lot of traffic that uses the lower port ranges, 
when using a PAT pool, you can now specify a flat range of 
ports to be used instead of the three unequal-sized tiers: 
either 1024 to 65535, or 1 to 65535.
We modified the following screens: Configuration > 
Firewall > NAT Rules > Add/Edit NAT Rule.
This feature is not available in 8.5(1) or 8.6(1).
Extended PAT for a PAT pool 8.4(3) Each PAT IP address allows up to 65535 ports. If 65535 
ports do not provide enough translations, you can now 
enable extended PAT for a PAT pool. Extended PAT uses 
65535 ports per service, as opposed to per IP address, by 
including the destination address and port in the translation 
information.
We modified the following screens: Configuration > 
Firewall > NAT Rules > Add/Edit NAT Rule.
This feature is not available in 8.5(1) or 8.6(1).
Table 5-1 Feature History for Twice NAT (continued)
Feature NamePlatform 
Releases Feature Information 
						

							 
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Automatic NAT rules to translate a VPN peer’s 
local IP address back to the peer’s real IP 
address8.4(3) In rare situations, you might want to use a VPN peer’s real 
IP address on the inside network instead of an assigned local 
IP address. Normally with VPN, the peer is given an 
assigned local IP address to access the inside network. 
However, you might want to translate the local IP address 
back to the peer’s real public IP address if, for example, 
your inside servers and network security is based on the 
peer’s real IP address.
You can enable this feature on one interface per tunnel 
group. Object NAT rules are dynamically added and deleted 
when the VPN session is established or disconnected. You 
can view the rules using the show nat command.
NoteBecause of routing issues, we do not recommend 
using this feature unless you know you need this 
feature; contact Cisco TAC to confirm feature 
compatibility with your network. See the following 
limitations:
Only supports Cisco IPsec and AnyConnect Client.
Return traffic to the public IP addresses must be 
routed back to the ASA so the NAT policy and VPN 
policy can be applied.
Does not support load-balancing (because of 
routing issues).
Does not support roaming (public IP changing).
ASDM does not support this command; enter the command 
using the Command Line Tool.
NAT support for IPv6 9.0(1) NAT now supports IPv6 traffic, as well as translating 
between IPv4 and IPv6. Translating between IPv4 and IPv6 
is not supported in transparent mode.
We modified the following screen: Configuration > Firewall 
> NAT Rules.
Table 5-1 Feature History for Twice NAT (continued)
Feature NamePlatform 
Releases Feature Information 
						

							 
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NAT support for reverse DNS lookups 9.0(1) NAT now supports translation of the DNS PTR record for 
reverse DNS lookups when using IPv4 NAT, IPv6 NAT, and 
NAT64 with DNS inspection enabled for the NAT rule.
Per-session  PAT 9.0(1) The per-session PAT feature improves the scalability of PAT 
and, for clustering, allows each member unit to own PAT 
connections; multi-session PAT connections have to be 
forwarded to and owned by the master unit. At the end of a 
per-session PAT session, the ASA sends a reset and 
immediately removes the xlate. This reset causes the end 
node to immediately release the connection, avoiding the 
TIME_WAIT state. Multi-session PAT, on the other hand, 
uses the PAT timeout, by default 30 seconds. For 
“hit-and-run” traffic, such as HTTP or HTTPS, the 
per-session feature can dramatically increase the 
connection rate supported by one address. Without the 
per-session feature, the maximum connection rate for one 
address for an IP protocol is approximately 2000 per 
second. With the per-session feature, the connection rate for 
one address for an IP protocol is 65535/average-lifetime.
By default, all TCP traffic and UDP DNS traffic use a 
per-session PAT xlate. For traffic that requires multi-session 
PAT, such as H.323, SIP, or Skinny, you can disable 
per-session PAT by creating a per-session deny rule.
We introduced the following screen: Configuration > 
Firewall > Advanced > Per-Session NAT Rules.
Table 5-1 Feature History for Twice NAT (continued)
Feature NamePlatform 
Releases Feature Information 
						

							 
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							CH A P T E R
 
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6
Configuring NAT (ASA 8.2 and Earlier)
This chapter describes Network Address Translation, and includes the following sections:
NAT Overview, page 6-1
Configuring NAT Control, page 6-16
Using Dynamic NAT, page 6-17
Using Static NAT, page 6-27
Using NAT Exemption, page 6-33
NAT Overview
This section describes how NAT works on the ASA, and includes the following topics:
Introduction to NAT, page 6-1
NAT in Routed Mode, page 6-2
NAT in Transparent Mode, page 6-3
NAT Control, page 6-4
NAT Types, page 6-6
Policy NAT, page 6-11
NAT and Same Security Level Interfaces, page 6-13
Order of NAT Rules Used to Match Real Addresses, page 6-14
Mapped Address Guidelines, page 6-14
DNS and NAT, page 6-14
Introduction to NAT
Address translation substitutes the real address in a packet with a mapped address that is routable on the 
destination network. NAT is composed of two steps: the process by which a real address is translated 
into a mapped address, and the process to undo translation for returning traffic.
The ASA translates an address when a NAT rule matches the traffic. If no NAT rule matches, processing 
for the packet continues. The exception is when you enable NAT control. NAT control requires that 
packets traversing from a higher security interface (inside) to a lower security interface (outside) match 
a NAT rule, or processing for the packet stops. See the “Security Levels” section on page 13-1 in the  
						

							 
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general operations configuration guide for more information about security levels. See the “NAT 
Control” section on page 6-4 for more information about NAT control.
NoteIn this document, all types of translation are referred to as NAT. When describing NAT, the terms inside 
and outside represent the security relationship between any two interfaces. The higher security level is 
inside and the lower security level is outside. For example, interface 1 is at 60 and interface 2 is at 50; 
therefore, interface 1 is “inside” and interface 2 is “outside.”
Some of the benefits of NAT are as follows:
You can use private addresses on your inside networks. Private addresses are not routable on the 
Internet. 
NAT hides the real addresses from other networks, so attackers cannot learn the real address of a 
host.
You can resolve IP routing problems such as overlapping addresses.
See Table 10-1 on page 10-4 for information about protocols that do not support NAT.
NAT in Routed Mode
Figure 6-1 shows a typical NAT example in routed mode, with a private network on the inside. When the 
inside host at 10.1.1.27 sends a packet to a web server, the real source address, 10.1.1.27, of the packet 
is changed to a mapped address, 209.165.201.10. When the server responds, it sends the response to the 
mapped address, 209.165.201.10, and the security appliance receives the packet. The security appliance 
then changes the translation of the mapped address, 209.165.201.10 back to the real address, 10.1.1.1.27 
before sending it to the host.
Figure 6-1 NAT Example: Routed Mode
Web Server
www.cisco.com
Outside
Inside209.165.201.2
10.1.2.1
10.1.2.27130023
Translation
209.165.201.10 10.1.2.27
Originating
Packet
Undo Translation
209.165.201.1010.1.2.27
Responding
PacketSecurity
Appliance