Hi,
Seems you are referring to source nat. You could configure a nat pool with that address. Example:
set security nat source pool test address x.x.x.x/x set security nat source rule-set snat from zone trust set security nat source rule-set snat to zone untrust set security nat source rule-set snat rule 1 match source-address 192.168.2.0/24 set security nat source rule-set snat rule 1 then source-nat pool test set security nat proxy-arp interface pp0.0 address x.x.x.x/x
The nat pool IP x.x.x.x has to be routed to this ppp0 WAN IP by the ISP.
Cheers,
Ashvin