def __dhcp_mac_to_chaddr(mac_addr='00:01:02:03:04:05'):
    """
    Private helper function to convert a 6-byte MAC address of form:
      '00:01:02:03:04:05'
    into a 16-byte chaddr byte string of form:
      '\x00\x01\x02\x03\x04\x05\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

    """
    chaddr = ''.join([chr(int(octet, 16)) for octet in mac_addr.split(':')])
    chaddr += '\x00' * 10
    return chaddr