Manual Chapter : Using NAT44 to Translate IPv4 Addresses

Applies To:

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  • 13.0.1, 13.0.0
Manual Chapter

Overview: NAT44

For the BIG-IP® system CGNAT module, NAT44 is the NAT type that maps IPv4 subscriber private addresses to IPv4 Internet public addresses. Translation addresses and ports are set in LSN pools. The CGNAT module performs NAT44 translations for all IP traffic.

diagram of a NAT44 network

Diagram of a NAT44 network

Task summary

About CGNAT hairpinning

An optional feature on the BIG-IP ®system, hairpinning routes traffic from one subscriber's client to an external address of another subscriber's server, where both client and server are located in the same subnet. To each subscriber, it appears that the other subscriber's address is on an external host and on a different subnet. The BIG-IP system can recognize this situation and send, or hairpin, the message back to the origin subnet so that the message can reach its destination.

Important: In order for hairpinning to function properly, the subscriber VLAN must be configured as an egress interface on the LSN pool. If the subscriber VLAN is not configured as an egress interface on the LSN pool, hairpinning fails.
Note: At present hairpinning works with all BIG-IP CGNAT scenarios except NAT64.

Creating an LSN pool

The CGNAT module must be enabled through the System > Resource Provisioning screen before you can create LSN pools.
Large Scale NAT (LSN) pools are used by the CGNAT module to allow efficient configuration of translation prefixes and parameters.
  1. On the Main tab, click Carrier Grade NAT > LSN Pools .
    The LSN Pool List screen opens.
  2. Click Create.
  3. In the Name field, type a unique name.
  4. In the Configuration area, for the Member List setting, type an address and a prefix length in the Address/Prefix Length field, and click Add.
    If your pool uses deterministic mode, ensure that any address ranges you enter as a member do not overlap another member's prefix address ranges. For example, the address and prefix overlaps
  5. Click Finished.
Your LSN pool is now ready, and you can continue to configure your CGNAT.

Creating a virtual server for an LSN pool

Virtual servers are matched based on source (client) addresses. Define a virtual server that references the CGNAT profile and the LSN pool.
  1. On the Main tab, click Carrier Grade NAT > Virtual Servers .
    The Virtual Server List screen opens.
  2. Click the Create button.
    The New Virtual Server screen opens.
  3. In the Name field, type a unique name for the virtual server.
  4. From the Type list, select Performance (Layer 4).
  5. For a network, in the Destination Address field, type an IPv4 or IPv6 address in CIDR format to allow all traffic to be translated.
    The supported format is address/prefix, where the prefix length is in bits. For example, an IPv4 address/prefix is, and an IPv6 address/prefix is ::/0.
  6. In the Service Port field, type * or select * All Ports from the list.
  7. From the VLAN and Tunnel Traffic list, select Enabled on. Then, for the VLANs and Tunnels setting, move the VLAN or VLANs on which you want to allow the virtual servers to share traffic from the Available list to the Selected list.
  8. For the LSN Pool setting, select the pool that this server will draw on for translation addresses.
  9. In the Resources area of the screen, for the iRules setting, select the name of the iRule that you want to assign and using the Move button, move the name from the Available list to the Enabled list.
  10. Click Finished.
The custom CGNAT virtual server now appears in the CGNAT Virtual Servers list.

Configuring an ALG profile

An ALG profile provides the CGNAT module with protocol and service information to make specified packet modifications to the IP and TCP/UDP headers, as well as the payload during translation.
Important: Edit only copies of the included ALG profiles to avoid unwanted propagation of settings to other profiles that use the included profiles as parents.
  1. On the Main tab, click Carrier Grade NAT > ALG Profiles .
  2. In the ALG Profiles menu, click an ALG profile.
  3. Click Create.
    The New Profile screen opens.
  4. Type a name for the new profile.
  5. From the Parent Profile list, ensure that the correct parent profile is selected as the new profile.
  6. Select the Custom check box on the right.
  7. Configure the profile settings.
  8. Click Finished to save the new ALG profile.
You now have an ALG profile for use by CGNAT.

Configuring a CGNAT iRule

You create iRules® to automate traffic forwarding for XML content-based routing. When a match occurs, an iRule event is triggered, and the iRule directs the individual request to an LSN pool, a node, or virtual server.
  1. On the Main tab, click Carrier Grade NAT > iRules .
    The iRule List screen opens.
  2. Click Create.
  3. In the Name field, type a 1 to 31 character name, such as cgn_https_redirect_iRule.
  4. In the Definition field, type the syntax for the iRule using Tool Command Language (Tcl) syntax.
    For complete and detailed information about iRules syntax, see the F5 Networks DevCentral web site (
  5. Click Finished.
You now have an iRule to use with a CGNAT virtual server.