import unittest
import ipaddress


class TestIpaddress(unittest.TestCase):

    def test_is_netmask_valid(self):
        ip = ipaddress._BaseV4()
        self.assertTrue(ip._is_valid_netmask('255.0.0.0'))
        self.assertTrue(ip._is_valid_netmask('255.128.0.0'))
        self.assertFalse(ip._is_valid_netmask('255.128.255.0'))
        self.assertFalse(ip._is_valid_netmask('255.128.255.255'))
        # this should pass, but failed
        self.assertFalse(ip._is_valid_netmask('255.254.128.0'))


if __name__ == '__main__':
    unittest.main()