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ASA NAT 8.3+ - NAT Operation and Configuration Format (CLI)


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My  name is Jouni Forss. I have worked in Finland at a local ISP for 5  years as a networking Engineer in a unit dedicated to servicing business  customers. My work for the most part consists of  managing/installing/developing Firewall and VPN environments that  include the use of Cisco PIXs , FWSMs and ASAs.

I participate at the Cisco Support Community in the Firewall and VPN section. And of those mainly at the Firewall section.

Version History

  • 20.3.2013 Initial version
  • 22.3.2013
    • Added Pictures attachement
    • Added NAT Configurations Text Files attachment

Possible Future Updates

The  Initial version of this document is supposed to provide some basic  information on the new 8.3+ NAT format. I will possibly add some more  sections to the document but the main point initially was to get some  version of this document out. If you want to get a notifications on the  changes to this document you can check the setting from the right side  panel of this page while logged in to the CSC.

Documents Purpose

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This  documents purpose will be to act as an informative document for users  new to ASA NAT configurations in general or just the New ASA NAT 8.3+  configurations format. The content directly reflects the things I run  into my day to day work or things that get asked on the CSC (Cisco  Support Community) Firewall and VPN sections.

The  document naturally wont contain every possible setup regarding the NAT  but more might be added later. Suggestions are welcome.

NAT Operation in ASA 8.3+

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The  new NAT format in 8.3 (and newer) software has introduced changes to  how the NAT rules are ordered in the ASA configurations. NAT  configurations are now divided into 3 different sections. The Section  determines the order of the NAT rules matched. Section 1 NAT  configurations are gone through first then Section 2 and finally Section  3


Rule Types

There are 2 NAT Rule Types

  • Twice NAT / Manual NAT
  • Network Object NAT

The  NAT Rule Types refer more to the configuration format of the NAT than  the actual type of the NAT (Dynamic PAT, Static PAT and so on). Below is  more information on both of the NAT Rule Types

Network Object NAT

Network Object NAT always consists of a "object network <object name>" configuration which holds a configuration for host address/subnet/range  and binds that to a NAT rule also present inside that same "object network". In other words all configuration related to the NAT configuration are gathered under a single "object network".

Notice that this is totally different than "object-group network <object-group name>". NAT configurations CANT BE configured under "object-group network"

This "object network <object name>" can be later referenced in other configurations with the name of the object. (ACL configurations)

Twice NAT / Manual NAT

Twice NAT / Manual NAT is not configured under any "object network" or "object-group network".

Instead Twice NAT / Manual NAT uses both "object network" and "object-group network" as its configuration parameter to define the source and destination  addresses/subnets/networks/ranges for your NAT configurations.

Twice NAT / Manual NAT also lets you utilize "object service <object name>" to manipulate the source and destination TCP/UDP ports in the NAT configurations.

The  key thing with Twice NAT / Manual NAT compared to Object Network NAT is  that you can manipulate both address/port source and destination  paremeters of the NAT. Therefore Twice NAT / Manual NAT gives you alot  more options than Network Object NAT.

Rule Types used per Section

The mentioned 3 Sections use different NAT Rule Types.

  • Section 1 uses Twice NAT / Manual NAT
  • Section 2 uses Network Object NAT
  • Section 3 uses Twice NAT / Manual NAT

Twice  NAT are by default inserted to the Section 1 of NAT rules on the ASA so  they are the first ones matched against traffic incoming to the ASA.  Network Object NAT rules are always inserted to the Section 2 of NAT  rules. Twice NAT rules configured with an "after-auto" parameter will be  moved to Section 3 of the NAT configuration and will therefore be the  last NAT rules matched on the ASA firewall.


NAT Types used with Twice NAT / Manual NAT and Network Object NAT

So far we know that NAT operates in 3 Sections and that each Section uses only certain Rule Type.

Now  we can have a look at which NAT Types are usually configure with each  NAT Rule Type. Take note that the below mentioned NAT Types per each NAT  Rule Type and the related commens inside "()" arent the absolute truth  on how you are supposed to configure NAT. It all depends on your  networks complexity among other things.

NAT Types of Network Object NAT (but NOT LIMITED to)
  • Static NAT
  • Static PAT (= Port Forward)
  • Dynamic Normal PAT (Usually done as Twice NAT in Section 3 instead of Section 2)
  • Dynamic Normal NAT (Usually done as Twice NAT in Section 3 instead of Section 2)
  • Dynamic Normal NAT+PAT (Usually done as Twice NAT in Section 3 instead of Section 2)
NAT Types of Twice NAT / Manual NAT (But NOT LIMITED to)
  • Dynamic Normal PAT (Used in Section 3)
  • Dynamic Normal NAT (Used in Section 3)
  • Dynamic Normal NAT+PAT (Used in Section 3)
  • Dynamic Policy PAT (Used in Section 1, possibly Section 3)
  • Dynamic Policy NAT (Used in Section 1, possibly Section 3)
  • Dynamic Policy NAT+PAT (Used in Section 1, possibly Section 3)
  • NAT0 / NAT Exemption / Identity NAT (Used in Section 1)
  • Static Policy NAT (Used in Section 1)
  • Static Policy PAT (Used in Section 1)

What  you have to notice regarding the Twice NAT / Manual NAT is the fact  that IT CAN BE USED IN BOTH SECTION 1 and SECTION 3. Part of the  mentioned NAT Types are therefore usually only used in Section 1 and  others only used in Section 3.

No use using a  NAT0/Identity NAT configuration in Section 3 when every other NAT rule  will possibly override it because of the order the NAT is processed.


Ordering of Rules Types Inside Sections

So  as mentioned in this document already, the Sections of the NAT  configurations already lay a foundation on what the Order/Priority of  the NAT configurations should be. In addtion to this each section has a  certain ordering of NAT rules.

Section 1 and Section 3 (Twice NAT / Manual NAT)

Twice NAT / Manual NAT has its own "line" parameter value that you use with the command.

It  operates/behaves the same way as ACL "line x" configurations in that it  moves the existing (on the line used) and any rule after it one line  number down wihtout removing any existing NAT configuration. Naturally  there is a chance that the configured rule will override some later rule  because it was inserted inbetween the existing configuration.

Opposed  to 8.2 (and below) software levels, this gives you the chance to insert  a NAT rule where want without the need to remove the existing NAT  configurations (Compare 8.2 Static NAT vs. Static Policy NAT between  same interfaces for example. If Static NAT has been configured first it  will always override the Static Policy NAT for the same interfaces)

Section 2 (Network Object NAT)

Network  Object NAT behaves more like the older 8.2 (and below) software. It has  a set order by which it decides what NAT rule to use. You cant  manipulate the order of the NAT rules with any kind of "line" value. The  only way to control the Section 2 Network Object NAT order is based on  how specific the NAT rules parameters are.

The Section  2 NAT however does have line number visible in some of the command  output BUT this value is determined by the ASA and as soon as you enter a  new Object Network NAT configuration the ASA calculates the new order  of the Network Object NAT rules.

Below is more on what factors into the priority of which Object Network NAT rule is used.

The first deciding factor in order is the NAT Type

  • Static
  • Dynamic

Inside the above mentioned NAT Types the following order applies

  • Amount  of IP addresses contained in "object  network"                                                                                                    
  • For "object network" containing same amount of IP addresses the lowest IP address number is first in order
  • For "object network" being equal on both above counts will be ordered by the alphabetical order of their names


NAT Configuration Structure and Considerations

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The Elements and Format of NAT Configurations

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This sections purpose is to go a bit more into the actual format and elements of the new NAT configurations.

From   the 8.2 (and older) NAT format we remember that that the three  commands  ("global", "nat" and "static") that form the basis of the NAT   configurations. Generally you would only use IP addresses or networks  as  the parameters of the NAT configuration. In special cases you would  use  "access-list" configurations to either define a NAT0 or Policy NAT   configuration.

In the software 8.3 (and newer) there   are no more ACLs in NAT configurations. You also very rarely refer to  an  actual IP address or network directly in the NAT configuration  line.  The new NAT format now utilizes "object network" , "object  service" and  "object-group network" to define the parameters of the  NAT  configuration. Naturally also the source and destination interface   "nameif" and the keyword "interface" will play a role.

Objects / Object-groups

As   mentioned above (and earlier in the document), the NAT configurations   now rely heavily on "object" and "object-group" configuration to  provide  the information to the actual NAT configuration.

You will be using the following as a parameter of Twice NAT / Manual NAT configurations

  • object-group network <NAME>                                                                                               
    • Used to define multiple networks, host addresses or combination of both in NAT configurations
  • object network <NAME>                                                                                              
    • Used to define a single subnet, address range or host address in NAT configurations
  • object service  <NAME>                                                                                            
    • Used to define source/destination services in NAT configurations

You can use the following as a parameter of Network Object NAT

  • object-group network <NAME>                                                                                                
    • Used to define multiple networks, host addresses or combination of both in NAT configurations
  • object network <NAME>                                                                                              
    • Used to define a single subnet, address range or host address in NAT configurations

And   I say you "CAN" use the "object" and "object-group network" even under  a  Object Network NAT but to this day I have still not done this. The   situation where you might do this is when you configure a Dynamic   NAT/PAT/NAT+PAT configuration as an Network Object NAT.

Naming Objects

Naming   the "object" and "object-group" objects in your configuration will  play  a big part in your NAT configurations. As almost every single one  of  your NAT rules will rely on some sort of object its good to come up  with  a policy for naming objects that will remain logical. It will save  you  time and possibly help in troubleshooting situations also.

My   personal preference is to use CAPS configuring the "object" or   "object-group" name. The most important reason for this is the fact  that  almost every command on the ASA uses lower case letters. In a NAT   configuration using CAPS means that you can read the CLI format   configuration more easily as the used "object" and "object-group" names   stand out better from the configurations.

The ASA  does  give you some options to rename objects/ACLs in the configuration  though  sadly this doesnt apply to every object type used in the NAT   configurations.

You can rename

  • object network
  • object service
  • access-list

With commands

  • object network <name> rename <newname>
  • object service <name> rename <newname>
  • access-list <name> rename <newname>

You CANT rename

  • object-group

Network Object NAT

This section will list the basic configuration format for the Network Object NAT

Static NAT & Static PAT

NON - Static NAT.jpg

NON - Static PAT Interface.jpg

NON - Static PAT IP.jpg

Dynamic PAT & Dynamic NAT & Dynamic NAT+PAT

NON - Dynamic PAT.jpg

NON - Dynamic NAT.jpg

NON - Dynamic NAT+PAT.jpg

Twice NAT / Manual NAT

This section will list the basic configuration format for the Twice NAT / Manual NAT

Dynamic PAT & Dynamic NAT & Dynamic NAT+PAT

TWICE - Dynamic PAT.jpg

TWICE - Dynamic NAT.jpg

TWICE - Dynamic NAT+PAT.jpg

Dynamic Policy PAT

TWICE - Dynamic Policy PAT.jpg

Dynamic Policy NAT

TWICE - Dynamic Policy NAT.jpg

Dynamic Policy NAT+PAT

TWICE - Dynamic Policy NAT+PAT.jpg

NAT0 / NAT Exemption / Indentity NAT

TWICE - NAT0 - NAT Exempt - Identity NAT.jpg

Static Policy NAT

TWICE - Static Policy NAT.jpg

Static Policy PAT

TWICE - Static Policy PAT.jpg

How to Utilize the NAT Sectioning

So far in this document we have discussed how the NAT configurations are structured in the new 8.3+ software levels.

We have seen that there is 3 Sections are processed in order

  • Section 1
  • Section 2
  • Section 3

We have seen that there is a Rule Type for each section

  • Section 1 - Twice NAT / Manual NAT
  • Section 2 - Network Object NAT
  • Section 3 - Twice NAT / Manual NAT

We have seen what NAT Types can be configured with each NAT Rule Type.

The  next question would be how to organize all this information so  configuring NAT on the ASA would be as clear as possible. I personally  configure NAT rules so that each Section serves a specific purpose  and/or specific users

Section 3 - The Default Dynamic Rules for Networks

The  NAT configurations located in Section 3 are the last ones to be matched  against a packet coming through the ASA. It seems only fitting to me  that this should be the Section where you build your most basic NAT  rules. Here you build the Dynamic PAT or Dynamic NAT or Dynamic NAT+PAT  for all the users so that they will have some "last resort" NAT when  they are connecting to networks past the ASA firewall.

As  Section 3 holds Twice NAT / Manual NAT type configurations you also  have the possibility to create destination based NAT rules so I would  also possibly consider creating Dynamic Policy PAT / NAT / NAT+PAT rules  for the users here.

In this case you will have to  make sure that you order the rules under the Section correctly. This  basically means that you should insert the Dynamic Policy PAT / NAT /  NAT+PAT rules for a certain pair of source/destination interfaces before  the the Default Dynamic PAT / NAT / NAT+PAT rule so it wont override  the Policy rule inside the same section.

Section 2 - The Default Static Rules for Single Hosts

The  NAT configurations located in Section 2 are matched against packet  coming through the ASA before the Section 3 rules. This makes it a  natural place to configure host specific NAT rules that you dont want to  fall into the Default NAT Rules.

As Section 2 holds  Network Object NAT rules, you dont have as much possibilities as with  Twice NAT / Manual NAT. For the most common Static NAT and Static PAT  configurations the Network Object NAT of Section 2 is more than enough  to meet the basic requirements for hosting services

Section 1 - The Special Dynamic/Static NAT Rules for Networks and Single Hosts

The  NAT configurations located in Section 1 are matched against packet  coming through the ASA before any of the other Sections. This makes  Section 1 the place where you will want to configure some rule that  needs to override any other rules you might have for the same  hosts/servers/networks. Naturally at the same time you will have to be  extra carefull in what you really define here because there is obviously  the highest risk of overriding something that you were not supposed to  override.

As Section 1 holds Twice NAT / Manual NAT  rules you manipulate both source and destination parameters of the NAT.  Section 1 would therefore be the section to use for example for NAT0 /  NAT Exempt / Identity NAT type configurations different Dynamic/Static  Policy NAT/PAT configurations.

Sections Combined

When    we combine the above defined roles of the different Sections we get   the  following general view of what NAT configurations we should use in   each  Section. Again, this is what I am used to doing and doesnt mean   this  wouldnt work in some other way.

  • Section 1
    • NAT0 / NAT Exemption
    • Policy PAT / NAT / NAT+PAT (for everything when Section 2 host based rules need to be overridden also)
    • Non-standard NAT configurations
  • Section 2
    • Static NAT
    • Static PAT (Port Forward)
  • Section 3
    • Default PAT / NAT / NAT+PAT
    • Policy PAT / NAT / NAT+PAT (for users)

Public IP Address Considerations

This  sections purpose is simply to suggest some things to consider regarding  the use of the public IP addresses you are given by the local ISP.  Wether they will help you naturally depends largely on how your ISP  functions and what it provides.

Single IP Address

  • Use Bridged WAN connection to the ASA when possible
  • Use Static Public IP address if possible                                                  
    • Ask for the possibility of MAC address binded DHCP if a Static Public IP address isnt possible otherwise
  • If forced to use DHCP consider using Dynamic DNS
  • Provide Internet access with Dynamic PAT and host local services with Static PAT

Probably the most common situation for any smaller company.

If  possible, try to get a bridged device to provide the Internet  connectivity to enable you to use the ISP provided public IP address  straight on your ASA. This will make related NAT configuration easier  and avoid the complexity and potential problems having a router in front  of ASA also doing NAT.

If you are planning on hosting  any services on a local server or using the ASA as a endpoint for VPN  connection, try to get a static public IP address from the ISP. If a  Static Public IP address isnt possible, consider asking for the  possiblity of binding a DHCP address to your devices MAC address

IP - Single IP.jpg

Small Subnet

Also  a pretty common setup. Usually a /29 (or public subnet  provided to you by the ISP. Remember that 3 of the 8 IP-addresses  provided by the /29 sized subnet are already taken from the start for  other purposes and you will have 5 IP address at your disposal.

For example

  • Network                                                                                                         
    • = Network Address
    • = Gateway (can naturally be some other IP address from the subnet)
    • = Broadcast Address
    • - = Usable Addresses on your ASA (one for ASA interface)

The  probably most common setup would be to use one public IP address as the  ASA "outside" interface IP address and also as the PAT IP address for  all outbound traffic from your LANs or DMZs. Other IP addresses could be  reserved for Static NAT use of servers. You should only resort to Port  Forward / Static PAT configurations if you know you wont have public IP  addresses for all your server needs.

IP - Small Subnet.jpg

Large Subnet

I  guess this is a relative term. I would already consider a /28  ( or /27 ( network a Large public  Network in our cases. Personally I dont see many bigger networks/subnets  handed out to business customers anymore. Only the bigger ones usually  have 1 - 3 /24 subnets.

When assigned with a /28, /27  or bigger network I would already consider splitting the networks into  2  to be used for different purposes and in 2 different places in your  network.

For example using one subnet for the link  between you and the ISP which can be utilized for Default PAT  configurations and possibly Static NAT configurations for server other  than the ones hosted on the DMZ of the ASA. Other segment could be used  direcly on the ASA DMZ interface or further on in your network on some  other L3 device which provides the gateway for the servers.

Though  naturally by segmenting an already relatively small subnet (even though  I call them large in this situation) means you are wasting some public  IP addresses as they will be deemed as network/gateway/broadcast  address.

The idea/reason with segmenting the public  subnets is that you can have the actual servers with the public IP  addresses without the need to resort to any special NAT setups on the  ASA side. You also can avoid problems related to DNS, especially when  you have a DNS server local to the users LAN.

IP - Large Subnet.jpg

Multiple Subnets

With  multiple discontinuous public subnets at our disposal I would also  suggest considering the same option as above. Using a subnet directly on  the DMZ segments to avoid any special needs regarding the NAT and DNS  while at the sametime using other subnet(s) directly on the ASA firewall  "outside" facing the ISP

IP - Multiple Subnets.jpg

One  thing to keep in mind with using multiple subnets on the interface  facing the ISP is that there have been changes from software version  8.4(2) -> 8.4(3) -> 8.4(4/5) in how the ASA operates with multiple  subnets on one interface. This mostly depends on how the ISP has  handled the routing of your public subnets.

If the ISP  has for example configure a new public subnet as a "secondary" network  on their gateway interface AND you are using 8.4(3) software you will  run into problems with connectivity of the hosts in the "secondary"  network range. This is because of changes to ARP related behaviour.  Basically the ASA will not populate ARP table with nonconnected  networks.

Your solution is either to ask the ISP to  route the new subnet directly towards the ASA "outside" interface IP  address OR you will have to upgrade the ASA to 8.4(4/5) software level  and use the configuration command "arp permit-nonconnected"

IP - Multiple Subnets ARP.jpg

Supporting Documentation

Cisco ASA Gonfiguration Guides

  • Contains example configurations
  • Provides background information/theory
  • Provides information about possible limitations

    Cisco ASA Command References

    • Contains more detailed information on different ASA configuration commands
    • Gives usage guidelines

    Cisco Support Community Firewall Discussion Area

    • Ask questions related to NAT from both Cisco Employees and Experts

    Final Word

    Hopefully  you have gotten some new information from this document and it has  helped you someway. If you have found it helpfull please do take the  time to rate it.

    Suggestion  about possible additions to the document are welcome. Please keep  specific question about how to configure some NAT configurations to the  actual Firewall section of CSC (link provided just above) If you happen  to find some error in the document that I have missed please let me know  and I will try to correct it.

    - Jouni


    just WOW! that was great ;)

    Lost & Found

    Hi Jouni,

    Thanks for the info. But i would like to which the best for this scenario for allowing specific address on nat while any other address are not allowed.




    Excellent Document

    Thank you very much !!


    I go through this document more often to refresh my ASA NAT knowledge. I must appreciate efforts put by Juoni. Thank you very much. 

    I have one query though-
    If I have 2 twice NAT statements
    1. Static PAT (For port forwarding)
    2. Identity NAT (For site-to-site VPN tunnel) 

    Considering execution of section 1 NAT rules, Traffic from internal server for which I have added 1st NAT rule, wont be able to pass through the tunnel because of rule 1 getting executed always. 

    Correct me If i am wrong? 

    VIP Advocate

    Thank you very much for this valued document. it helps me alot on day to day jobs on firewall natting.

    thank you very much.


     Excellent document,  Thank you very much!!

    Community Member
    Thank you very much!
    Community Member

    Rite now am working in ASA 5505 firewall,

    blue phoenix


    Thanks for this!

    What is the order of operation then if the traffic is from a lower security level to a higher security level?

    lower security interface > higher security interface
    2.twice nat
    source IP (unchanged) > destination IP (translated or changed)

    Would the return traffic need also an ACL allowing
    higher security interface > lower security interface
    source IP (translated) > source IP (changed back to mapped IP) destination IP

    If you need further details, please just ask.



    The inbound-acl applied to lower-security interface will reference the real destination-IP not the mapped-IP.

    So the order: NAT(untranslate), consult acl and if allowed, route to real-destination.

    Return traffic doesn't need to be explicitly allowed via a separate inbound-acl on higher security interface.

    Pre-8.3 implementations needed the mapped-destination-IP

    Order: consult acl and if mapped-destination-IP is allowed, untranslate to real-dest-IP and route.

    First of all, I want to thank Jouni Forss for posting such useful document. I came across it due I was looking for HOW TO APPLY ACLs to NAT/PAT on ASA. I am quite familiar with IOS, so I was thinking that it would be like in IOS. I mean, in IOS NAT/PAT is defined by ACLs. I am wondering, is there more option than applying ACLs to the interfaces for packet filtering for NAT/PAT on ASA?



    Screenshot from 2017-10-25 17-43-19.png




    I need to do NAT source & destination at the same time i.e Which means when a packets goes frm inside to outside, source changes to is the WAN IP (for source ANY) and destination changes to (for destination matching 172.17.0.x/24) and vice versa.

    Which diagram should i refer to? Sorry i m beginner.

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