Netopia R5100, R5200, R5300 Netopia Router

C-4 User’s Reference GuideWhen the Netopia R5000 Series Router receives this IP packet from the WWW server, the Netopia R5000 Series Router replaces the destination IP address with 192.168.5.2, the address for Workstation A. The port is changed back to 400, the IP packet checksum is recalculated, and the IP packet is sent to Workstation A on the Netopia R5000 Series Router’s LAN interface.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 R5000 Series Router 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 (por t 80), for example.Now look at how two hosts on the LAN interface accessing the same WWW server on the Internet will work:NetopiaRouter

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