Netopia R5300 - 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 ser ver 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
por t could be 400 while the destination por t would be 80 (WWW ser ver).
When the Netopia R5000 Series Router receives this IP packet, it cannot simply for ward it to the WAN inter face
and the Internet since the IP addresses on the LAN inter face 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 inter face 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 inter face, which in this case is 200.1.1.40. Next the Netopia R5000 Series Router will
substitute the source TCP or UDP por t with a TCP or UDP por t 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 inter face to its destination, the WWW ser ver 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 por t is substituted with 5001, then the IP packet
checksum is recalculated. When this modified packet reaches the WWW ser ver on the Internet, the WWW
ser ver responds and sends the IP packet back to destination IP address 200.1.1.40 and destination por t
5001.
Netopia Router
WWW Server
163.176.4.32 ISP Router
200.1.1.1
ISP Router to WWW
Src IP: 200.1.1.40
Dst IP: 163.176.4.32
Src Port: 5001
Dst Port: 80
WWW to 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
Netopia Router
LAN: 192.168.5.1
WAN: 200.1.1.40 Workstation A
192.168.5.2