Following are available source address translation types and the typical use case for each.
Dynamic IP and Port
For a given source IP address, the Palo Alto Networks firewall translates the source IP address or range to a single IP address. The mapping is based on source port, so multiple source IPs can share a single translated address until the source ports have been exhausted. This is typical when only having a single public IP address to be shared among many private IP addresses. It is common to choose the IP address assigned to the interface connecting to your ISP:
To add more IP addresses to the outbound pool, change the address type to "Translated Address" and add a valid public IP to the list. The firewall will load balance from the address pool based on each session.
Use the following CLI command to check the NAT pool utilization: > show running global-ippool
For a given source IP address, the firewall translates the source IP to an IP in the defined pool or range. The mapping is not port based, which makes this a one-to-one mapping as long as the session lasts. Each concurrent session uses an address from the pool, making it unavailable to other source IPs. Be aware when using this option, because the translated pool of addresses can be exhausted if the number of internal hosts concurrently creating outbound sessions exceeds the number of IP addresses in the dynamic pool. This option is used when there are two or more public IPs from the ISP, but not enough to allocate one to each internal host on the network, and you want to assign them to outbound hosts only as needed. It is common to assign a range of IP addresses to the dynamic pool:
To view the current NAT pool mappings for a given NAT policy, run the following CLI command:
> show running nat-rule-ippool rule <NAT rule name>
Use this translation type to translate a single source address to a specific public address. This is typically used to expose a server (email, web or any application) externally using a translated address that will not change.
Selecting "Yes" for Bi-directional creates mapping in both directions based on the source\destination zones that are specified. If Bi-directional is set to No, then the mapping is created based only on the direction of the source\destination zones. Static NAT policies for publicly exposed servers usually have Bi-directional set to Yes, so the outbound traffic for the server uses the same address as inbound traffic:
Use the Static IP mapping type to translate an entire address range to a specific address range, a one-to-one mapping. The number of source IPs using this policy must exactly match the translated range. This is typically used to resolve overlapping IP ranges when merging networks. The policy shown here translates all source addresses with at 10.20.1.x address destined to the Corp Zone to a matching address in the 10.30.1.x range: