IP Pools

IP Pools are a mechanism that allow sessions leaving the FortiGate Firewall to use NAT. An IP pool defines a single IP address or a range of IP addresses to be used as the source address for the duration of the session. These assigned addresses will be used instead of the IP address assigned to that FortiGate interface.

caution icon When using IP pools for NATing, there is a limitation that must be taken into account. In order for communication to be successful in both directions, it is normal for the source address in the packet header assigned by the NAT process to be an address that is associated with the interface that the traffic is going through. For example, if traffic is going out an interface with the IP address, packets would be NATed so that the source IP address would be This way the returning traffic will be directed to the same interface on the same FortiGate that the traffic left from. Even if the packets are assigned a source address that is associated with another interface on the same FortiGate this can cause issues with asymmetrical routing. It is possible to configure the NATed source IP address to be different than the IP address of the interface but you have to make sure that the routing rules of the surrounding network devices take this unorthodox approach into consideration.

There are 4 types of IP Pools that can be configured on the FortiGate firewall:

  • One-to-One - in this case the only internal address used by the external address is the internal address that it is mapped to.
  • Overload - this is the default setting. Internal addresses other than the one designated in the policy can use this address for the purposes of NAT.
  • Fixed Port Range - rather than a single address to be used, there is a range of addresses that can be used as the NAT address. These addresses are randomly assigned as the connections are made.
  • Port Block Allocation - this setting is used to allocate a block of port numbers for IP pool users. Two variables will also have to be set. The block size can be set from 64 to 4096 and as the name implies describes the number of ports in one block of port numbers. The number of blocks per user determines how many of these blocks will be assigned. This number can range from 1 to 128.
caution icon Be careful when calculating the values of the variables. The maximum number of ports that are available on an address is 65,536. If you chose the maximum value for both variables you will get a number far in excess of the available port numbers.

4096 x 128 = 524,288

One of the more common examples is when you have an email server behind your FortiGate firewall and the range of IP addresses assigned to you by your ISP is more than one. If an organization is assigned multiple IP addresses it is normally considered a best practice to assign a specific address other than the one used for the Firewall to the mail server. However, when normal NAT is used the address assigned to the firewall is also assigned to any outbound sessions. Anti-spam services match the source IP address of mail traffic that they receive to the MX record on DNS servers as an indicator for spam. If there is a mismatch the mail may not get through so there is a need to make sure that the NATed address assigned matches the MX record.

You can also use the Central NAT table as a way to configure IP pools.

Source IP address and IP pool address matching when using a range

When the source addresses are translated to an IP pool that is a range of addresses, one of the following three cases may occur:

Scenario 1:

The number of source addresses equals that of IP pool addresses

In this case, the FortiGate unit always matches the IP addressed one to one.

If you enable fixed port in such a case, the FortiGate unit preserves the original source port. This may cause conflicts if more than one security policy uses the same IP pool, or the same IP addresses are used in more than one IP pool.

Scenario 2:

The number of source addresses is more than that of IP pool addresses

In this case, the FortiGate unit translates IP addresses using a wrap-around mechanism. If you enable fixed port in such a case, the FortiGate unit preserves the original source port. But conflicts may occur since users may have different sessions using the same TCP 5 tuples.

Scenario 3:

The number of source addresses is fewer than that of IP pool addresses

In this case, some of the IP pool addresses are used and the rest of them are not be used.

ARP Replies

If a FortiGate firewall interface IP address overlaps with one or more IP pool address ranges, the interface responds to ARP requests for all of the IP addresses in the overlapping IP pools. For example, consider a FortiGate unit with the following IP addresses for the port1 and port2 interfaces:

  • port1 IP address: (range is
  • port2 IP address: (range is

And the following IP pools:

  • IP_pool_1:
  • IP_pool_2:
  • IP_pool_3:

The port1 interface overlap IP range with IP_pool_1 is:

( and ( =

The port2 interface overlap IP range with IP_pool_2 is:

( & ( =

The port2 interface overlap IP range with IP_pool_3 is:

( & ( =

And the result is:

  • The port1 interface answers ARP requests for
  • The port2 interface answers ARP requests for and for

Select Enable NAT in a security policy and then select Dynamic IP Pool. Select an IP pool to translate the source address of packets leaving the FortiGate unit to an address randomly selected from the IP pool. Whether or not the external address of an IP Pool will respond to an ARP request can be disabled. You might want to disable the ability to responded to ARP requests so that these address cannot be used as a way into your network or show up on a port scan.

IP pools and zones

Because IP pools are associated with individual interfaces IP pools cannot be set up for a zone. IP pools are connected to individual interfaces.

Fixed Port

Some network configurations do not operate correctly if a NAT policy translates the source port of packets used by the connection. NAT translates source ports to keep track of connections for a particular service.

However, enabling the use of a fixed port means that only one connection can be supported through the firewall for this service. To be able to support multiple connections, add an IP pool, and then select Dynamic IP pool in the policy. The firewall randomly selects an IP address from the IP pool and assigns it to each connection. In this case, the number of connections that the firewall can support is limited by the number of IP addresses in the IP pool.


The match-vip feature allows the FortiGate unit to log virtual IP traffic that gets implicitly dropped. This feature eliminates the need to create two policies for virtual IPs; one that allows the virtual IP, and the other to get proper log entry for DROP rules.

For example, you have a virtual IP security policy and enabled the match-vip feature; the virtual IP traffic that is not matched by the policy is now caught.

The match-vip feature is available only in the CLI. By default, the feature is disabled.