Network Address Translation

When you send data to someone, the packets need to store the Source IP address as well as the destination IP address. The Source's IP address, however, will be the private IP address. i.e. 192.168.x.x Therefore, the Network Address Translation changes the Source IP address into the public IP address before sending the packet on to the internet.

The router will replace your private IP address with its own public IP address. This translation is recorded so that incoming data addressed to the router can redirected to the originating private IP.

Further explanation:

https://www.youtube.com/watch?v=AkmQZx1ffKg&list=PLCiOXwirraUCw0BWhWk_5rInOWRnzf8Xm&index=12