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C-4 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 R7200 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:
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 R7200 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.
Netopia
Router
WWW Server
163.176.4.32
ISP Router
200.1.1.1
Netopia Router
LAN: 192.168.5.1
WAN: 200.1.1.40
Workstations
A & B
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
WWW to ISP Router
Src IP: 163.176.4.32
Dst IP: 200.1.1.40
Src Port: 80
Dst Port: 5002
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
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