Simple answer, if NAT to one source address from multiple real source addresses would not work, no network of more than 1 host would be able to surf the internet. Right?
The kind of SNAT that is needed in this case is the "overload" NAT, which is the default. Configured as "one-to-one" NAT will not work, as Vishal has already mentioned.
How to: configure an IP pool containing just the one desired translated address (type "overload", external IP range "a.b.c.d - a.b.c.d"), enable NAT in the outbound policy but choose to specify the address to use. Then select the IP pool just created.
The NAT which you have enabled is one-to-one mapping. It works like this, suppose you have configured a NAT type <one-to-one> and in the range if you have configured like this : 188.8.131.52-184.108.40.206 then it works on first come and first serve basis.
When traffic from OriginSrcIP1 to AnyDest it will be translated to the IP 220.127.116.11.
Now when at the same time if another traffic came OriginSrcIP2 to AnyDest then the device will not be able to NAT the IP because in the range you have given one-to-one 18.104.22.168-22.214.171.124.
It will work only when the IP in the NAT pool is free.
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