Overload with single port allocation CGN IP pool

On the GUI go to Policy & Objects > IP Pools > Create > IP Pool. Set IP Pool Type to IPv4 IP Pool, set Type to CGN Resource Allocation, and set Mode to Overload (Single Port Allocation). You can enable NAT64 to make this a NAT64 IP pool.

On the CLI:

config firewall ippool

edit <name>

set type cgn-resource-allocation

set startip <ip>

set endip <ip>

set arp-reply {disable | enable}

set arp-intf <interface-name>

set cgn-spa enable

set cgn-overload enable

set cgn-port-start <port>

set cgn-port-end <port>

set utilization-alarm-raise <usage-threshold>

set utilization-alarm-clear <usage-threshold>

set nat64 {disable | enable}

set exclude-ip <ip>, <ip>, <ip> ...

end

An overload single port allocation CGN resource allocation IP pool assigns single ports instead of ranges of ports. This type of CGN IP pool conserves ports by effectively reducing the port block size to 1. Since this is an overload IP pool, ports are re-used. A client session can get any port from the range of ports added to the IP pool that are available.

Since blocks of ports are not assigned to each client and ports are re-used, there are no limits on the number of ports that a client IP address can use. Port re-use is determined by how much the pool is utilized. This IP pool type works for networks with large numbers of clients where those clients may start many individual sessions.

You can define an overload single port allocation IP pool by configuring the following:

• External IP range (start-ip and end-ip). Specifies the set of translation IP addresses available in the pool as a collection of IP prefixes with their prefix lengths. These are typically public-side addresses.
• Start port (cgn-port-start). The lowest port number in the port range. The default value is 5117.
• End port (cgn-port-end). The highest possible port number in the port range. The default value is 65530
• Enable or disable ARP reply (arp-reply) to reply to ARP requests for addresses in the external address range.
• Optionally specify the interface (arp-intf) that replies to ARP requests.
• Generate an SNMP trap when the usage of the resources defined by an IP pool exceeds a threshold (utilization-alarm-raise). The range is 50 to 100 per cent.
• Generate an SNMP trap when the usage of the resources defined by an IP pool falls below this threshold (utilization-alarm-clear). The range is 40 to 100 per cent.
• You can enable nat64 to make this a NAT64 IP pool.
• Exclude specific IP addresses (exclude-ip). Specify external IP addresses that the CGN IP pool will not allocate. This is a security feature that allows you to exclude one or more IP addresses from being allocated if the IP pool could assign addresses that have been targeted by external attackers. You can only add single IP addresses. You cannot add IP address ranges. Use the ? to see how many IP addresses you can add. The limit depends on the FortiGate model.