If you have Mac OS X 10.4, then I've found that the easiest way to set up port forwarding on the Airport Extreme/Express is to use a fixed IP address for your computer and use the NAT Port Mapping Protocol. First, ensure that your ADSL modem is not used as a router (many people have BOTH their router and Airport setup as a router. For example, both the Zoom 5660 and DSL 300T are ADSL modem /routers: ensure that they are set in to a mode such as "bridge": essentially, this means that your Airport Express/Extreme RECEIVES the IP address that your ADSL provider gives you.)
Then, to set up the Airport for port forwarding, do this:
1. Log into Airport Admin Utiity, and choose the Network tab. Choose Distribute IP addresses, and Share a SINGLE IP address. Then choose use 172.16.1.1 addressing.
2. Under the Airport tab, choose Base Station Options. Tick the following:
Enable SNMP Access
Enable Remote Configuration
Enable Default Remote Host at 172.16.1.253
Enable NAT Port Mapping Protocol
3. Goto the Internet tab to make a note of the DNS servers (these are for your ADSL provider: they should be in grey to the right of the screen: if they are not, do not worry.)
Save the configuration.
Your internet connection won't work now: you need to give your computer the fixed IP address chosen above. To do this:
4. Goto System Preferences, Network, then TCP/IP.
Your connection will probably be set up Automatically using DHCP.
First, make a note of the Subnet Mask (will probably be 255.255.255.0)
Then, change the Configure IPv4 tag to: Manually
IP Address: 172.16.1.253
Subnet Mask: (whatever you noted above, eg: 255.255.255.0)
Router: 172.16.1.1 (this is the Airport's IP Address)
DNS Servers: your ADSL provider's servers (from part 3: if you do not have them, phone your ADSL provider, or try using: 4.2.2.1 and 4.2.2.2).
5. Save the settings (Apply Now) then goto Show All and Sharing, then choose the Firewall tab.
Ensure the Firewall is ON, then choose New... to choose a new Port (eg: 9090).
Update and restart your computer.
