import ipaddress


def subnet_prefix_valid(subnet_prefix: str) -> bool:
    """
    Determines whether or not a subnet prefix is in the following format:

        x.x.x.x/xx

    :param str subnet_prefix: the subnet prefix to validate

    :return: True if valid, falise otherwise

    """
    try:
        ipaddress.ip_network(subnet_prefix)
        return True
    except ValueError:
        pass
    return False