code: def cidr_to_ipv4_netmask(cidr_bits):
      '''
      Returns an IPv4 netmask
      '''
      try:
          cidr_bits = int(cidr_bits)
          if not 1 <= cidr_bits <= 32:
              return ''
      except ValueError:
          return ''
  
      netmask = ''
      for idx in range(4):
          if idx:
              netmask += '.'
          if cidr_bits >= 8:
              netmask += '255'
              cidr_bits -= 8
          else:
              netmask += '{0:d}'.format(256 - (2 ** (8 - cidr_bits)))
              cidr_bits = 0
      return netmasf cidr_to_ipv4_netmask(cidr_bits):
      '''
      Returns an IPv4 netmask
      '''
      try:
          cidr_bits = int(cidr_bits)
          if not 1 <= cidr_bits <= 32:
              return ''
      except ValueError:
          return ''
  
      netmask = ''
      for idx in range(4):
          if idx:
              netmask += '.'
          if cidr_bits >= 8:
              netmask += '255'
              cidr_bits -= 8
          else:
              netmask += '{0:d}'.format(256 - (2 ** (8 - cidr_bits)))
              cidr_bits = 0
      return netmask
docs: Returns an IPv4 netmask