My colleage used ZyXEL Prestige 650R-31 as the main ADSL router for his company. It works very well and so stable. Last week he asked me to help him setup a server placed behide the ADSL router to be accessible anywhere from Internet. The main service is e-mail so I just added port forwarding via web-based configuration. It was so easy and worked like a charm. However, the connection could not establish if he sat inside the NAT. The problem is so called "NAT loopback". I found this kind of problem in D-Link also.

Fortunately, ZyXEL allows to enable NAT loopback via commandline interface. What I do to solve this problem is just 4 steps.

1. Telnet to the router and enter administrative password
2. Go to menu 24 and then 8
3. Run command "ip nat loopback on"
4. Type "exit" and then 99 to quit from the management screen

Thanks ZyXEL and cpbotha for this very useful hidden command. It would be better if it can be altered via web interface.

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