Understanding Netopia NAT Behavior F-3
When the Netopia R3100 receives this IP packet, it can not 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 R3100 has to change the IP packet to reflect the IP address that was acquired on the WAN interface from the ISP.
The Netopia R3100 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 R3100 will substitute the source TCP or UDP port with a TCP or UDP port from within a specified range maintained within the Netopia R3100. 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:
WWW Server | ISP Router | Netopia Router | Workstation A | |||||
LAN: 192.168.5.1 | ||||||||
163.176.4.32 | 192.168.5.2 | |||||||
200.1.1.1 | WAN: 200.1.1.40 | |||||||
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Router
Netopia
ISP Router to WWW Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port:: 5001 Dst Port:: 80
WWWto ISP Router Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port:: 80
Dst Port:: 5001
Netopia to ISP Router Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port:: 5001
Dst Port:: 80
ISP Router to Netopia Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port:: 80
Dst Port:: 5001
Wkstn A to Netopia Src IP: 192.168.5.2 Dst IP: 163.176.4.32 Src Port:: 400
Dst Port:: 80
Netopia to Wkstn A Src IP: 163.176.4.32 Dst IP: 192.168.5.2 Src Port:: 80
Dst Port:: 400
As you can see, the IP packet from Workstation A is sent to the Netopia R3100 and 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.
When the Netopia R3100 receives this IP packet from the WWW Server, the Netopia R3100 replaces the destination IP address with 192.168.5.2, the address for Workstation A. The port is changed back to 400, the IP packet checksum is recalculated, and the IP packet is sent to Workstation A on the Netopia R3100s LAN interface.
The reasons for the IP address changes are obvious from the diagram above but what is not so obvious is why the TCP or UDP source ports need to be changed as well. These are changed and maintained in an internal table so the Netopia R3100 can determine which host on the local LAN interface sent the IP packet and what host the response from the WAN interface is going to go to on the LAN interface. This becomes especially important when two or more hosts on the LAN interface are accessing the same type of service on the Internet, like a WWW Server (Port 80), for example.
Now look at how two hosts on the LAN interface accessing the same WWW Server on the Internet will work:
