code: def _check_nm(nm, notation):
      
      # Convert to decimal, and check if it's in the list of valid netmasks.
      _NM_CHECK_FUNCT = {
          NM_DOT: _dot_to_dec,
          NM_HEX: _hex_to_dec,
          NM_BIN: _bin_to_dec,
          NM_OCT: _oct_to_dec,
          NM_DEC: _dec_to_dec_long}
      try:
          dec = _NM_CHECK_FUNCT[notation](nm, check=True)
      except ValueError:
          return False
      if dec in _NETMASKS_VALUES:
          return True
      return False
docs: Function internally used to check if the given netmask
      is of the specified notation.