Make Life easy for yourself.
Leave everything the way it is right now. Make sure your client computer is set to use DHCP. Click on Advanced –> Ethernet and write down the Mac Address for the client/server computer…
- Open Airport Utility…..
- Open your Airport Extreme
- Click on Internet
- Click on DHCP
- Click on + under DHCP Reservations
- Add a description (eg Server), and click on Reserve Address By Mac Address
- Click on Continue
- Add the Mac Address, and type in the IP Address you want to assign it.
This will cause the computer to always get the same IP Address through DHCP. You are literally reserving the IP for that computers use.