Set Static IP Reservations on AirPort Extreme
According to the fourth hit on Google here, all you need to do is open get the MAC address of your server, then open AirPort Utility and select your AirPort base station and choose edit (or just click on it if you're not using version 6.0+) and choose the Network tab. Seeing as how you have different local addresses, I'm assuming you have DHCP enabled (it's always a good idea) you just need to set up an IP reservation for the MAC address of your server.