code: def _is_valid_netmask(self, prefixlen):
          
          try:
              prefixlen = int(prefixlen)
          except ValueError:
              return False
          return 0 <= prefixlen <= self._max_prefixlen
docs: 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.