code: def _is_valid_netmask(self, 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
docs: 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.