Netopia R910 manual C-190 User’s Reference Guide, Workstations

C-190 User’s Reference Guide

The reasons for the IP address changes are obvious from the preceding diagram, 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 R910 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:

		Netopia Router
WWW Server	ISP Router	LAN: 192.168.5.1	Workstations
163.176.4.32	200.1.1.1	WAN: 200.1.1.40	A & B

Router

Netopia

ISP Router to WWW Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port: 5001

Dst Port: 80

ISP Router to WWW Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port: 5002

Dst Port: 80

WWWto ISP Router Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port: 80

Dst Port: 5002

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

Netopia to ISP Router Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port: 5002

Dst Port: 80

ISP Router to Netopia Src IP: 163.176.4.32 Dst IP: 200.1.1.40 Src Port: 80

Dst Port: 5002

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

Wkstn B to Netopia Src IP: 192.168.5.3 Dst IP: 163.176.4.32 Src Port: 400

Dst Port: 80

Netopia to Wkstn B Src IP: 163.176.4.32 Dst IP: 192.168.5.3 Src Port: 80

Dst Port: 400

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, when Workstation A and Workstation B transmit an IP packet to the WWW server on the Internet, they have unique source IP addresses on the LAN interface but potentially the same source ports, which in this case is 400. When the Netopia R910 receives these packets, the source IP addresses are substituted with the single globally unique IP address that was acquired on the WAN interface, which is 200.1.1.40.