[whoami_nr]

One of the early projects I worked to teach myself networking was Port Knocking[1]. Think of it like designing a calling bell for your servers where they can smartly understanding who is doing the "knocking" on their ports. Writing it as a linux kernel module, with the low level cryptographic functions would be a great learning tool imo. I also wrote about my progress here[2] when I attempted this about 7-8 years ago.

[1]:https://en.wikipedia.org/wiki/Port_knocking [2]:https://rnikhil.com/2016/12/12/port-knocking-python.html

[whoami_nr]

You can also get your hands dirty with writing a proxy or a VPN which in my opinion also exposes you to a lot of low level stuff. My other suggestion would be malware reverse engineering but I don't think you are interested in systems/OS based on your post.