Verify that the netmask is valid.

        Args:
            netmask: A string, either a prefix or dotted decimal
              netmask.

        Returns:
            A boolean, True if the prefix represents a valid IPv4
            netmask.

def _is_valid_netmask(self, netmask):
        """Verify that the netmask is valid.

        Args:
            netmask: A string, either a prefix or dotted decimal
              netmask.

        Returns:
            A boolean, True if the prefix represents a valid IPv4
            netmask.

        """
        mask = netmask.split('.')
        if len(mask) == 4:
            try:
                for x in mask:
                    if int(x) not in self._valid_mask_octets:
                        return False
            except ValueError:
                # Found something that isn't an integer or isn't valid
                return False
            for idx, y in enumerate(mask):
                if idx > 0 and y > mask[idx - 1]:
                    return False
            return True
        try:
            netmask = int(netmask)
        except ValueError:
            return False
        return 0 <= netmask <= self._max_prefixlen