validate an IPv6 integer netmask

def __ipv6_netmask(value):
    '''validate an IPv6 integer netmask'''
    valid, errmsg = False, 'IPv6 netmask (0->128)'
    valid, value, _ = __int(value)
    valid = (valid and 0 <= value <= 128)
    return (valid, value, errmsg)