D-4 User’s Reference Guide
WWW Server |
| Netopia Router | Workstations | |||||
ISP Router | LAN: 192.168.5.1 | |||||||
163.176.4.32 | A & B | |||||||
200.1.1.1 | WAN: 200.1.1.40 | |||||||
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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 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:
