def _is_valid_netmask(self, prefixlen):
try:<EOL><INDENT>prefixlen = int(prefixlen)<EOL><DEDENT>except ValueError:<EOL><INDENT>return False<EOL><DEDENT>return <NUM_LIT:0> <= prefixlen <= self._max_prefixlen<EOL>
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.