Understanding Netopia NAT Behavior D-3

When the Netopia R2121 receives this IP packet, it can not simply forward it to the WAN interface and the Internet since the IP addresses on the LAN interface are not valid or globally unique for the Internet. Instead the Netopia R2121 has to change the IP packet to reflect the IP address that was acquired on the WAN interface from the ISP.

The Netopia R2121 will first substitute the source IP address with the IP address that was acquired on the WAN interface which in this case is 200.1.1.40. Next the Netopia R2121 will substitute the source TCP or UDP port with a TCP or UDP port from within a specified range maintained within the Netopia R2121. And finally the modified IP packet's checksum is recalculated (as specified in RFC 1631) and the packet is transmitted across the WAN interface to its destination, the WWW Server on the Internet.

If the send and response IP packets were drawn out, this process would look like the following:

WWW Server

ISP Router

Netopia Router

Workstation A

LAN: 192.168.5.1

163.176.4.32

192.168.5.2

200.1.1.1

WAN: 200.1.1.40

 

 

 

 

 

 

Router

Netopia

ISP Router to WWW Src IP: 200.1.1.40 Dst IP: 163.176.4.32 Src Port:: 5001 Dst Port:: 80

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

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

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, the IP packet from Workstation A is sent to the Netopia R2121 and the source IP address is substituted with 200.1.1.40 and the source port is substituted with 5001, then the IP packet checksum is recalculated. When this modified packet reaches the WWW Server on the Internet, the WWW Server responds and sends the IP packet back to destination IP address 200.1.1.40 and destination port 5001.

When the Netopia R2121 receives this IP packet from the WWW Server, the Netopia R2121 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 R2121s LAN interface.

The reasons for the IP address changes are obvious from the diagram above 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 R2121 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:

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Netopia R2121 manual Router Netopia
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R2121 specifications

The Netopia R2121 is a powerful and versatile router designed for both home and office environments, providing seamless connectivity and robust performance for various networking needs. This device offers a range of features and technologies that cater to the demands of modern users seeking reliable internet access, speed, and security.

One of the standout characteristics of the Netopia R2121 is its dual-band technology, which operates on both the 2.4 GHz and 5 GHz frequency bands. This capability allows users to enjoy faster speeds and less interference, as the 5 GHz band is typically less congested than the traditional 2.4 GHz band. With the ability to support multiple devices simultaneously, the R2121 ensures that users can stream videos, play online games, and conduct video conferences without experiencing lag or connectivity issues.

The router is equipped with advanced security features, including WPA3 encryption, which provides enhanced protection against unauthorized access and data breaches. The R2121 also supports a robust firewall system that guards against external threats, ensuring that the user's network remains secure. Additionally, the router includes parental controls, allowing users to manage and restrict internet access for specific devices or set time limits for usage, making it an excellent choice for families.

Another significant advantage of the Netopia R2121 is its extensive coverage area. With high-gain antennas and advanced beamforming technology, the router can deliver strong and stable Wi-Fi signals even in larger homes or office spaces. This technology focuses the Wi-Fi signal directly toward connected devices, minimizing dead zones and improving overall connectivity.

The R2121 also supports both IPv4 and IPv6, ensuring compatibility with current and future internet technologies. This forward-thinking design makes it a future-proof solution for users looking to invest in a reliable router.

In terms of setup and management, the Netopia R2121 features a user-friendly web interface that allows users to easily customize settings, monitor network performance, and manage connected devices. The inclusion of mobile app support further enhances the user experience, enabling remote monitoring and control of the network from anywhere.

Overall, the Netopia R2121 stands out as a robust and reliable router that combines advanced features, enhanced security, and excellent performance, making it an ideal choice for home and business users alike. With its focus on delivering seamless connectivity and comprehensive management options, the R2121 is well-suited for today's connected lifestyles.
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