'Return the length of an subnet.
>>> print IP(\'195.185.1.0/28\').len()
16
>>> print IP(\'195.185.1.0/24\').len()
256'

def len(self):

    if (self._ipversion == 4):
        locallen = (32 - self._prefixlen)
    elif (self._ipversion == 6):
        locallen = (128 - self._prefixlen)
    else:
        raise ValueError('only    IPv4    and    IPv6    supported')
    return (2 ** locallen)
