def validate_mac_address(mac_address):
    """Validate MAC address & return it in correct format"""
    mac_address = mac_address.lower()
    for digit in mac_address:
        if digit in ".:-":
            mac_address = mac_address.replace(digit, "")
    if len(mac_address) != 12:
        return None
    for digit in mac_address:
        if digit not in [
            "0",
            "1",
            "2",
            "3",
            "4",
            "5",
            "6",
            "7",
            "8",
            "9",
            "a",
            "b",
            "c",
            "d",
            "e",
            "f",
        ]:
            return None
    mac_address = mac_address[0:4] + "." + mac_address[4:8] + "." + mac_address[8:12]
    return mac_address