Step 4: Router Runaround

Now set your Mac's IP address to match the designated default host address, so that when you enter your Base Station's or router's IP address (from step 3), the router automatically forwards the traffic to the default host Mac. Open System Preferences > Network, and select Built-In Ethernet from the Show menu. Select Manually from the Configure IPv4 menu. Enter 10.0.1.253 in the IP Address field, leave the Subnet Mask at 255.255.255.0, and enter the router's IP address on your local network. AirPort Extreme uses 10.0.1.1; Belkin and D-Link routers use 192.167.0.1; consult your router's manual for help. Finally, enter a DNS Server or two-get some from your ISP.

The router expects a specific IP address, which you can force on any connected Mac.