Well, if the number of bits that compose the network portion are not supplied, then we can't calculate the netmask.  In this case, the code will throw an exception since [1] is out of bounds.  I thought about putting in a guard and throwing an exception in this case, but since the end result is that an exception will be thrown regardless, it seemed best to just let Ruby do its thing instead of writing code that does it.  Though I could make it throw a named exception instead.