Netopia R5000 User Reference Manual page 255

Understanding Netopia NAT Behavior C-3
With NAT enabled, the Netopia R5000 Series Router does something different. For example, suppose that
Workstation A again wants to communicate with the WWW server on the Internet. Workstation A forms an IP
packet with the source IP address of 192.168.5.2 and destination IP address of 163.176.4.32, and source
port could be 400 while the destination port would be 80 (WWW server).
When the Netopia R5000 Series Router receives this IP packet, it cannot simply forward it to the WAN interface
and the Internet since the IP addresses on the LAN interface are not valid or globally unique for the Internet.
Instead, the Netopia R5000 Series Router has to change the IP packet to reflect the IP address that was
acquired on the WAN interface from the ISP.
The Netopia R5000 Series Router will first substitute the source IP address with the IP address that was
acquired on the WAN interface, which in this case is 200.1.1.40. Next the Netopia R5000 Series Router will
substitute the source TCP or UDP port with a TCP or UDP port from within a specified range maintained within
the Netopia R5000 Series Router. And finally the modified IP packet's checksum is recalculated (as specified in
RFC 1631) and the packet is transmitted across the WAN interface to its destination, the WWW server on the
Internet.
If the send and response IP packets were drawn out, this process would look like the following:
As you can see, the IP packet from Workstation A is sent to the Netopia R5000 Series Router, the source IP
address is substituted with 200.1.1.40 and the source port is substituted with 5001, then the IP packet
checksum is recalculated. When this modified packet reaches the WWW server on the Internet, the WWW
server responds and sends the IP packet back to destination IP address 200.1.1.40 and destination port
5001.