'Verify that the netmask/prefixlen is valid.
Args:
prefixlen: A string, the netmask in prefix length format.
Returns:
A boolean, True if the prefix represents a valid IPv6
netmask.'

def _is_valid_netmask(self, prefixlen):

    try:
        prefixlen = int(prefixlen)
    except ValueError:
        return False
    return (0 <= prefixlen <= self._max_prefixlen)
