Netopia R2121 User Reference Manual page 246

D-4 User's Reference Guide
WWW Server
163.176.4.32
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
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 R2121 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 R2121 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.
If you were to look at the internal port mapping table that is maintained by the Netopia R2121, it would look
similar to the following:
ISP Router
200.1.1.1
Router
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
Netopia Router
LAN: 192.168.5.1
WAN: 200.1.1.40
Netopia
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
Workstations
A & B