Even with Security Zones in use, one must still define all networks behind each interface for purposes of antispoofing enforcement. For any network that not "flat" (i.e. has additional routed networks beyond the VLAN the firewall is physically attached to) this will typically be represented as a specific group. Those same interface antispoofing groups could be used in manual NAT rules to approximate the effect of Security Zones, but if groups containing a large number of objects are placed into both the source and destination of a manual NAT rule, that can expand out to a very large number of individual NAT rules so watch out. Representing the Internet here can be a bit tricky too, essentially you have to use a group with exclusion in the destination, which can also cause some unexpectedly large expansions.
Example: a group with 100 networks is added to the source of a manual NAT rule, and another group with 100 networks is added to the destination. During policy compilation that will expand out to 10,000 individual NAT rules. In the old days that value could get high enough to cause a policy compilation failure.
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