Discover Devices

I am only able to discover devices 255 at a time. I set my net mask to 255.255.0.0 and should be able to search for:

start : 45.0.0.0
end : 45.0.255.255

Right?

Well it only picks up 45.0.0.0 to 45.0.0.255.

Does anyone know what is going on?

Thanks.