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Suppose endpoint in public network (WAN) accesses endpoint in private network (LAN) through NAT (assume some pre-defined mapping exists so NAT knows how to route incoming request). Is it possible for NAT to change source IP address of packets coming from endpoint in WAN before transmitting the packets to the endpoint in private network? Or only destination is being changed?

I have only found "overlapping" mode, but this is very rare condition.

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2 Answers 2

Normally NAT only changes the destination IP-address on incoming packets and the source IP-address on outgoing packets. This applies for port-forwarding too - which is the name for routing incoming requests via a NAT router to a specific host.

If I understand correctly, Overlapping NAT is only needed if your internal LAN is illegitimately using a public IP address range that is registered to another company. This is what NAT and the private address ranges were designed to avoid.

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I wouldn't call that NAT as such but certainly it's very possible. We use Zeus's ZXTMs to do something very similar actually. Of course it would depend on what you already have in your infrastructure, let us have more details and we'll see what can be added.

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