Although there is more abstract software to manage firewalls, like ufw on debian-based systems and firewall-cmd on redhat systems, Iptables will help you understand what actually happens during filtering, mangling or routing a package. https://www.frozentux.net/iptables-tutorial/iptables-tutorial.html has a structured approach in explaining what happends when a package hits the firewall. Pay extra attention to Network Address Translation. Here is another nice HOWTO: https://netfilter.org/documentation/HOWTO/NAT-HOWTO-5.html
Please have a look at http://www.robinkrens.nl/tunneling.html +
Please have a look at ./tunneling.html:tunneling.html