def get_mac_str(valve_index, port_num):
    """Gets the mac address string for the valve/port combo
    Args:
        valve_index (int): The internally used id of the valve.
        port_num (int): port number

    Returns:
        str
    """
    two_byte_port_num = ("%04x" % port_num)
    two_byte_port_num_formatted = two_byte_port_num[:2] + ':' + two_byte_port_num[2:]
    return '00:00:00:%02x:%s' % (valve_index, two_byte_port_num_formatted)