It is recommended that the clients' end routers be blocked so as not to allow the client to see the ISP network.

The use of NAT (Network Address Translation) on client routers is common practice and generally considered appropriate in many situations.

However, whether this is appropriate or not depends on the specific context and customer network requirements.

We explain the reasons why NAT is used and some considerations to determine if it is the best option:

Reasons to use NAT

1. Privacy & Security: NAT helps hide a network's internal IP addresses, which can offer an additional layer of security by making it more difficult for external attackers to target specific devices within the network.
2. Conservation of IP addresses: Since IPv4 addresses are limited, NAT allows multiple devices to share a single public IP address. This is crucial for organizations that cannot obtain a sufficient range of public IP addresses for each device.
3. Simplicity and management- NAT can simplify network management by allowing administrators to configure internal networks without worrying about potential IP address conflicts on the broader Internet.

Considerations for using NAT

1. Connectivity issues: NAT can complicate certain setups, such as VPNs, VoIP, and online gaming, where end-to-end communication and port mapping are essential.
2. Performance: Depending on how it is configured and the capability of the device, NAT can introduce latency and reduce network performance if the router is not adequately equipped to handle the traffic loads.
3. IPv6: With the adoption of IPv6, which provides virtually unlimited address space, the use of NAT could become less necessary in the long term, as each device can have its own public address.
4. Transparency and traceability: User activities may be more difficult to trace specifically to a device when they all share the same public IP address, which can be an issue for certain auditing or compliance requirements.

Conclusion

If customer routers are performing NAT, this is generally acceptable and in line with standard practices, especially on IPv4 networks where IP addresses are scarce. However, it is important to ensure that any NAT implementation does not compromise necessary network functionality, security, or performance.

Additionally, as the world moves toward IPv6, it could be beneficial to review and possibly adjust NAT strategies to align with new capabilities and practices.