input: Write a python function to complete the IPv4 address by adding missing octets with '0'.
  assert ipv4_format('192.168.1') == '192.168.1.0'
  assert ipv4_format('10.0') == '10.0.0.0'
  assert ipv4_format('172.16') == '172.16.0.0'
  assert ipv4_format('192') == '192.0.0.0'
  assert ipv4_format('255.255.255.255') == '255.255.255.255'
output: def ipv4_format(ip):
      octets = ip.split('.')
      while len(octets) < 4:
          octets.append('0')
      return '.'.join(octets)