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E-4 User’s Reference Guide
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 R310 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.
Now both IP packets have the exact same source IP address (200.1.1.40) and source ports (400). The way the
Netopia R310 is then able to distinguish between the two IP packets is to change the source TCP or UDP ports
and keep this information in an internal table. As seen above, the source port for Workstation A has been
changed to 5001 and the source port for Workstation B has been changed to 5002.
Netopia
Router
Wkstn A to Netopia
Src IP: 192.168.5.2
Dst IP: 163.176.4.32
Src Port:: 400
Dst Port:: 80
Workstations
A & B
Netopia Router
LAN: 192.168.5.1
WAN: 200.1.1.40
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 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
ISP Router to Netopia
Src IP: 163.176.4.32
Dst IP: 200.1.1.40
Src Port:: 80
Dst Port:: 5001
Netopia to Wkstn A
Src IP: 163.176.4.32
Dst IP: 192.168.5.2
Src Port:: 80
Dst Port:: 400
ISP Router
200.1.1.1
WWW Server
163.176.4.32
Wkstn B to Netopia
Src IP: 192.168.5.3
Dst IP: 163.176.4.32
Src Port:: 400
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 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
ISP Router to Netopia
Src IP: 163.176.4.32
Dst IP: 200.1.1.40
Src Port:: 80
Dst Port:: 5002
Netopia to Wkstn B
Src IP: 163.176.4.32
Dst IP: 192.168.5.3
Src Port:: 80
Dst Port:: 400