import ipaddress


def cidr_to_netmaskv6(cidr):
    """Creates a decimal format of a CIDR value.

    Args:
        cidr (int): A CIDR value.

    Returns:
      netmask (str): Decimal format (IPv6) representation of CIDR value.

    Example:
        >>> from netutils.ip import cidr_to_netmaskv6
        >>> cidr_to_netmaskv6(24)
        'ffff:ff00::'
        >>> cidr_to_netmaskv6(17)
        'ffff:8000::'
    """
    if isinstance(cidr, int) and 0 <= cidr <= 128:
        return str(ipaddress.IPv6Address(((1 << cidr) - 1) << (128 - cidr)))
    raise ValueError("Parameter must be an integer between 0 and 128.")