Publish Sharepoint for internet

I have sharepoint 2007 server, it has sites. Using local computer within the same network I can access the portal easily with no problem.
I enabled the firewall to allow the outsider users from access the portal using port 80. Now, when I try to access the portal from outside (internet), then I will be prompted for a user name and password, but then I get page cannot be displayed!
What should I do to enable the outsider from accessing the portal?

I am using FortiGate firewall and MOSS 2007.

thanks
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