You're a victim of what's known as NAT Loopback Blocking. This is fixable by running your own internal DNS server, optionally with a DHCP server
Step 1: A fast introduction to NATs
NATs are often used to turn non-routable networks such as 10.0.0.0/8 and 192.168.0.0/24 (both common networks) into routable IPs (often referred to as "Public" or "External" IPs.)
NAT Loopback is a term used when a NAT understands a connection request for its External IP from an Internal IP and routes it as though it were a request from the outside, heeding your particular port-forwarding requests.
Many ISP-provided routers disallow this, and for good reason: Its really easy to fake a NAT loopback, sending the 'external' connection through a loop or just outright denying it.
Step 2: Installing Debian [part 1]
I'm going to install Debian Squeeze (Testing) in a virtual machine, however the steps are going to be mostly the same given a real box.
The first step is getting a Network Install CD burned. You can get them from http://cdimage.debian.org -- You will find links to all the appropriate locations.