Router – Zyxel P-320W: How to connect to the web server using public IP

Remember, when your firewall/router/whatever is doing NATing, the internal box that has a public IP NATed to it doesn't know anything about the NATed address.

So, in order for this to work, your packet has to leave your internal LAN, go out to the external network, then come back in via the firewall/router/whatever that's doing the NATing, and finally be handed to the internal box (which, again, doesn't generally know anything about the NATing).

There are some work-arounds that are possible (see Cisco alias command, if you're using Cisco), but in general it's not really something you want to do.

This is why most networks that contain internal hosts and externally accessible (public) hosts will maintain separate DNS servers for internal and external hosts. You can setup your external DNS server so that www.example.com points to your external IP, and setup your internal DNS server so that www.example.com point so your internal IP. If you're using BIND, then views make this quite easy.

You want to use the external IP in order to access the machine whether you're internal or not, correct?

While this isn't very efficient, you might have to dink with your DNS records to create an entry for your "public" IP to point to the internal server. That way you'll be able to always hit the server whether you're inside the network or not. It sounded like you already had port forwarding working (so you could access the server from the outside using the public IP and when internal you could use the internal IP), which is what the firewall configuration would do using port forwarding.

The solution I'm proposing would mean you have your own DNS server, and if not, you would have to set up a DNS server to host your internal records and proxy everything else out for DNS lookups.