This page lists some useful resources for a more in depth understanding on specific subjects. Assumed is that you have a basic understanding of Linux and Networking. If not, you might to start with one of the followings books +
+A good way to understand more about networking is two setup two computers: a server and a client. And the play around with the tools. The following tools and documentation are extremely useful. +
+Simple tool to open or connect to TCP or UDP ports and output data through these channels. Build and test proxies. Powerful for debugging. Cryptcat is a similar tool, but with support for cryptography +
+Create and send IP, TCP or UDP packages. You are able to edit any value within these packages. +
+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 ./tunneling.html:tunneling.html +
+Here are some good cheatsheets for commonly used tools +
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