def is_valid_ip6_prefix(ip6_prefix):
    """Validates given string as IPv6 prefix.

    Args:
        ip6_prefix (str): string to validate as IPv6 prefix.

    Returns:
        bool: True if string is valid IPv6 prefix, else False.
    """
    if not ip6_prefix.isdigit():
        return False

    ip6_prefix_int = int(ip6_prefix)

    if ip6_prefix_int < 0 or ip6_prefix_int > 128:
        return False

    return True