I am a bit irritated that "iptables" belongs not to the solution, because in a google search I found out that iptables is "allowing and blocking various services by port, network interface, and source IP address.".
Thank you for your feed back.
The intent of the question was to recognize that "netfilter" is the packet filtering framework built into the kernel and that "iptabes" is a userspace program used to configure "netfilter". Other programs can also configure netfilter like firewalld,gfw and the YAST Firewall command.
This will be clarified in the next release of the course materials.