In a default single WAN connection configuration, pfSense® software automatically generates NAT rules for every directly connected interface and any networks reachable through internal gateways defined via Static Routes </routing/static-routes>. These NAT rules will perform NAT on outbound traffic to translate the source address of Internet-bound traffic to the WAN IP.
In a default multi-WAN configuration, pfSense software automatically generates NAT rules to NAT any internal subnet leaving any interface with a gateway.
The Outbound NAT Configuration may be changed by visiting Firewall > NAT on the Outbound tab.
With Manual Outbound NAT </nat/advanced-outbound-nat>, this automatic rule generation may be disabled and custom outbound NAT rules may be defined manually. When switching to Manual Outbound NAT without any rules defined, a set of rules is created automatically that is the equivalent of the rules currently in use by Automatic Outbound NAT.