4.9.4.8 Network Address Translation (NAT)

Question 1

How does NAT work?

Answer 1

The source address of the packet can’t be the local IP address on the LAN, it has to be the external one.
Before the packet is sent to the ISP, the router swaps the source address (local address) in the IP Packet for an address from the NAT pool (public address).
The IP packet then traverses the Internet and hits the destination device. The reply is formed and the return IP packet is generated. The sources address the packet came in with (the public address from the NAT pool) now becomes the destination address for the return journey. The public address is routable and thus the packet is routed back to the edge router of the network. The packets public destination address is swapped for the local address. The packet now has a destination address of the original sources and is returned to the originator.
NAT allocates a unique port to be stored alongside the device’s local IP that sent the packet originally, so that the router knows which device requested the packet.

Question 2

Explain what NAT is and why it is performed

Answer 2

Modifying network / IP addresses (and port numbers); as they pass through a router;
The private IP addresses used on the LAN are not routable, so cannot be used for routing on the Internet;
The private IP addresses used on the LAN are not unique, so cannot be used by the host to return data;
To improve the security of the LAN;