Change behavior for the network-address and broadcast-address.
Users should not specify such addresses, but if they do, generate
something more sensible.

Also, if the address was in network larger then /24, the
generated address range was rather unexpected. Change behavior
here.

There are no particularly strong reasons for the chosen range.
It just seems suitable. The decision to hand out at most a /24
is because it is likely to be plenty, and because that is what
the previous code did -- at least, if the address was in the
first /24 of the subnet. See how the result for 192.168.0.1/20
is unchanged, but 192.168.1.1/20 changes.