Returns the "natural" mask of an IPv4 address

def natural_ipv4_netmask(ip, fmt='prefixlen'):
    '''
    Returns the "natural" mask of an IPv4 address
    '''
    bits = _ipv4_to_bits(ip)

    if bits.startswith('11'):
        mask = '24'
    elif bits.startswith('1'):
        mask = '16'
    else:
        mask = '8'

    if fmt == 'netmask':
        return cidr_to_ipv4_netmask(mask)
    else:
        return '/' + mask