Is it possible to change the default gateway of the DHCP server side of an Airport Extreme I will try to be brief. In my career I have always used the upper end of the subnet for network devices. For instance, I use .250-.254 for network devices such as the routers. I am very fond of using .254 as the default gateway for internal networks. Is there a way to change the private network default gateway of the Airport Extreme to x.x.x.254? As an example, the factory default network is 10.0.1.0/255.255.255.0 with a default gateway of 10.0.1.1. I would like to change the gateway to 10.0.1.254. 

By the way, I think this community has put together a fantastic site.
Just configure the AP in Bridge-mode so it's running DHCPD and distributing addresses.

Then, on the internet tab configure it statically and give it the high IP you want. You probably want to configure the range to be 1-253.

Personally, I find Airport Utility 5.6 to be better than the 6+ versions. You might want to give that a try too (see here for how to get it).