code: def natural_ipv4_netmask(ip, fmt='prefixlen'):
      '''
      Returns the "natural" mask of an IPv4 address
      '''
      bits = _ipv4_to_bits(ip)
  
      if bits.startswith('11'):
          mask = '24'
      elif bits.startswith('1'):
          mask = '16'
      else:
          mask = '8'
  
      if fmt == 'netmask':
          return cidr_to_ipv4_netmask(mask)
      else:
          return '/' + masf natural_ipv4_netmask(ip, fmt='prefixlen'):
      '''
      Returns the "natural" mask of an IPv4 address
      '''
      bits = _ipv4_to_bits(ip)
  
      if bits.startswith('11'):
          mask = '24'
      elif bits.startswith('1'):
          mask = '16'
      else:
          mask = '8'
  
      if fmt == 'netmask':
          return cidr_to_ipv4_netmask(mask)
      else:
          return '/' + mask
docs: Returns the "natural" mask of an IPv4 address