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NAT in Production

To better understand how NAT works, let’s look at the example in the image below and find out what happens when PC1 pings the server on the internet. The packet exits PC1 with a source IP of 192.168.1.10 (inside local), and after NAT, it is replaced with a public IP of 203.0.113.1 (inside global).

Topology of NAT in production identifying inside/outside local/global areas

In the meantime, the destination IP of the server 209.165.202.129 (outside local and outside global IPs are the same by default) stays the same in the Layer 3 header. When the server replies to PC1, the source IP of that packet is 209.165.202.129, while the destination IP is 203.0.113.1.

Upon receipt of the packet, the router performs NAT based on the data in the NAT table, translates the destination IP from 203.0.113.1 to 192.168.1.10, and sends the packet to PC1.