def parse_mac(value):
    raw = value.strip().lower().replace(":", "").replace("-", "")
    if len(raw) != 12:
        raise ValueError("address must be 12 hex chars or aa:bb:cc:dd:ee:ff")
    try:
        return bytes.fromhex(raw)
    except ValueError:
        raise ValueError("address contains non-hex characters")


def format_mac(mac_bytes):
    return ":".join("{:02x}".format(b) for b in mac_bytes)