NAT in Transparent Mode

Similar to operating in NAT mode, when operating a FortiGate unit in Transparent mode you can add security policies and:

  • Enable NAT to translate the source addresses of packets as they pass through the FortiGate unit.
  • Add virtual IPs to translate destination addresses of packets as they pass through the FortiGate unit.
  • Add IP pools as required for source address translation

A FortiGate unit operating in Transparent mode normally has only one IP address – the management IP. To support NAT in Transparent mode, you can add a second management IP. These two management IPs must be on different subnets. When you add two management IP addresses, all FortiGate unit network interfaces will respond to connections to both of these IP addresses.


Use the following steps to configure NAT in Transparent mode:

1. Add two management IPs

2. Add an IP pool to the WAN1 interface

3. Add an Internal to WAN1 security policy

You can add the security policy from the web-based manager and then use the CLI to enable NAT and add the IP pool.

The usual practice of NATing in transparent mode makes use of two management IP addresses that are on different subnets, but this is not an essential requirement in every case.

If there is a router between the client systems and the FortiGate unit you can use the router’s capabilities of tracking sessions to assign NATed addresses from an IP pool to the clients even if the assigned address don’t belong to a subnet on your network.



Client computer has an IP address of on the subnet

Router “A” sits between the client computer and the FortiGate (in Transparent mode) with the IP address of on the client’s side of the router and the IP address of on the FortiGate’s side of the router.

Use NAT to assign addresses from an address pool of to to traffic coming from gateway of

To enable the return traffic to get to the original computer, set up a static route than assigns any traffic with a destination of to go through the gateway. As long as the session for the outgoing traffic has been maintained, communication between the client computer and the external system on the other side of the FortiGate will work.


Central NAT Table

The central NAT table enables you to define, and control with more granularity, the address translation performed by the FortiGate unit. With the NAT table, you can define the rules which dictate the source address or address group and which IP pool the destination address uses.

While similar in functionality to IP pools, where a single address is translated to an alternate address from a range of IP addresses, with IP pools there is no control over the translated port. When using the IP pool for source NAT, you can define a fixed port to guarantee the source port number is unchanged. If no fix port is defined, the port translation is randomly chosen by the FortiGate unit. With the central NAT table, you have full control over both the IP address and port translation.

The FortiGate unit reads the NAT rules in a top-down methodology, until it hits a matching rule for the incoming address. This enables you to create multiple NAT policies that dictate which IP pool is used based on the source address. The NAT policies can be rearranged within the policy list as well. NAT policies are applied to network traffic after a security policy.


NAT 64 and NAT46

NAT64 and NAT46 are the terms used to refer to the mechanism that allows IPv6 addressed hosts to communicate with IPv4 addressed hosts and vice-versa. Without such a mechanism an IPv6 node on a network such as a corporate LAN would not be able to communicate with a web site that was still in a IPv4 only environment and IPv4 environments would not be able to connect to IPv6 networks.

One of these setups involves having at least 2 interfaces, 1 on an IPv4 network and 1 on an IPv6 network. The NAT64 server synthesizes AAAA records, used by IPv6 from A records used by IPv4. This way client-server and peer to peer communications will be able to work between an IPv6 only client and an IPv4 server without making changes to either of the end nodes in the communication transaction. The IPv6 network attached to the FortiGate unit should be a 32 bit segment, (for instance 64:ff9b::/96, see RFC 6052 and RFC 6146). IPv4 address will be embedded into the communications from the IPv6 client.

Because the IPv6 range of addresses is so much larger than the IPv4 range, a one to one mapping is not feasible. Therefore the NAT64 function is required to maintain any IPv6 to IPv4 mappings that it synthesizes. This can be done either statically by the administrator or automatically by the service as the packets from the IPv6 network go through the device. The first method would be a stateless translation and the second would be a stateful translation. NAT64 is designed for communication initiated from IPv6 hosts to IPv4 addresses. It is address mapping like this that allows the reverse to occur between established connections. The stateless or manual method is an appropriate solution when the NAT64 translation is taking place in front of legacy IPv4 servers to allow those specific servers to be accessed by remote IPv6-only clients. The stateful or automatic solution is best used closer to the client side when you have to allow some specific IPv6 clients to talk to any of the IPv4-only servers on the Internet.

There are currently issues with NAT64 not being able to make everything accessible. Examples would be SIP, Skype, MSN, Goggle talk, and sites with IPv4 literals. IPv4 literals being IPv4 addresses that are imbedded into content rather than a FQDN.


Policies that employ NAT64 or NAT46 can be configured from the web-based manager as long as the feature is enabled using the Features setting found at System > Config > Features.

  • To create a NAT64 policy go to Policy > Policy > NAT64 Policy and select Create New.
  • To create a NAT46 policy go to Policy > Policy > NAT46 Policy and select Create New.


The difference between these NAT policies and regular policies is that there is no option to use the security profiles and sensors.


NAT 66

NAT 66 is Network Address Translation between 2 IPv6 network. The basic idea behind NAT 66 is no different than the regular NAT between IPv4 networks that we are all used to. The difference are in the mechanics of how it is performed, mainly because of the complexity and size of the addresses that are being dealt with.

In an IPv4 world, the reason for the use of NAT was usually one or a combination of the following 3 reasons:

  • Improved security – actual addresses behind NAT are virtually hidden
  • Amplification of addresses – hundreds of computers can use as little as a single public IP address
  • Internal address stability – there is control of internal addressing. The addresses can stay the same even if Internet Service Providers change.

In these days of security awareness the protective properties of NAT are not something that are not normally depended on by themselves to defend a network and with the vastly enlarged IPv6 address scope there is no longer a need to amplify the available addresses. However, the desire to have internal address control still exists. The most common reason for using NAT66 is likely to be the maintaining of the existing address scheme of the internal network despite changes outside of it. Imagine that you have an internal network of 2000 IP addresses and one day the company changes its ISP and thus the addresses assigned to it. Even if most of the addressing is handled by DHCP, changing the address scheme is going to have an impact on operations.


Addressing stability can be achieved by:

  • Keeping the same provider – this would depend on the reason for the change. If the cost of this provider has become too expensive this is unlikely. If the ISP is out of business it becomes impossible.
  • Transfer the addresses from the old provider to the new one – There is little motivation for an ISP to do you a favor for not doing business with them.
  • Get your own autonomous system number – this can be too expensive for smaller organizations.
  • NAT – this is the only one on the list that is in the control of IT.

There are differences between NAT66 and IPv4 NAT. Because there is no shortage of addresses most organizations will be given a /48 network that can be translated into another /48 network. This allows for a one to one translation, no need for port forwarding. This is a good thing because port forwarding is more complicated in IPv6. In fact, NAT66 will actually just be the rewriting of the prefix on the address.



If your current IPv6 address is 2001:db8:cafe::/48 you could change it to 2001:db8:fea7::/48

There is an exception to the one to one translation. NAT66 cannot translate internal networks that contain 0xffff in bits 49 through 63 – this is due to the way checksums are calculated in TCP/IP: they use the one’s-complement representation of numbers which assigns the value zero to both 0x0000 and 0xffff.

Having trouble configuring your Fortinet hardware or have some questions you need answered? Check Out The Fortinet Guru Youtube Channel! Want someone else to deal with it for you? Get some consulting from Fortinet GURU!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.