Cannot Access Server from External IP (Router)

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http://serverfault.com – We have an Ubuntu 10.04 LAMP server running on site (Apache is on port 80). It is running fine through the LAN, but I cannot access it using its external IP address through the Netopia 3000 Series modem by which it is connected to the internet. I have tried using what Netopia calls "Pinholes" (Port Forwarding) to make the server accessible, but a telnet to the external address just gives me "connect to address x.x.x.x: Operation timed out." I have also tried enabling "Services" and "IP Passthrough" on the router, but it gives the same result as above. How can I enable the server to be accesse (HowTos)