how do I see IP addresses of connected devices to my airport extreme

I'm trying to reuse a Net Gear N600 router I had to shelve since I installed my Airport Extreme router as an access point. I need to see other IP addresses so I can configure the N600 IP address as 10.0.1.xxx. The only way to make sure I don't use an already used IP address for the N600 is to see what IP addresses are used by the system.

The only support I've found here is how to set up a distributed network using other apple airport extreme devices.

Has anyone done/added a second non-apple router configured as an access point to an airport extreme. If you have please let me know what you did. I intend to have the N600 connected via ethernet cable to my airport extreme to extend both my wifi network and connected devices.

Thanks,

Steven