Dynamic IP and Port (DIPP) NAT allows you to use each translated IP address and port pair multiple times (8, 4, or 2 times) in concurrent sessions. This reusability of an IP address and port (known as oversubscription) provides scalability for customers who have too few public IP addresses. The design is based on the assumption that hosts are connecting to different destinations, therefore sessions can be uniquely identified and collisions are unlikely. The oversubscription rate in effect multiplies the original size of the address/port pool to 8, 4, or 2 times the size. For example, the default limit of 64K concurrent sessions allowed, when multiplied by an oversubscription rate of 8, results in 512K concurrent sessions allowed.

The oversubscription rates that are allowed vary based on the model. The oversubscription rate is global; it applies to the firewall. This oversubscription rate is set by default and consumes memory, even if you have enough public IP addresses available to make oversubscription unnecessary. You can reduce the rate from the default setting to a lower setting or even 1 (which means no oversubscription). By configuring a reduced rate, you decrease the number of source device translations possible, but increase the DIP and DIPP NAT rule capacities. To change the default rate, see Modify the Oversubscription Rate for DIPP NAT.

If you select Platform Default, your explicit configuration of oversubscription is turned off and the NAT default DIPP pool oversubscription rate for the model applies, as shown in the Product Selection tool. The Platform Default setting allows for an upgrade or downgrade of a software release.

The firewall supports a maximum of 256 translated IP addresses per NAT rule, and each model supports a maximum number of translated IP addresses (for all NAT rules combined). If oversubscription causes the maximum translated addresses per rule (256) to be exceeded, the firewall will automatically reduce the oversubscription ratio in an effort to have the commit succeed. However, if your NAT rules result in translations that exceed the maximum translated addresses for the model, the commit will fail.