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I found interactive CIDR visualization tools like https://cidr.xyz/ to be very helpful in understanding the notation.

I also end up using https://gitlab.com/ipcalc/ipcalc a lot, and am definitely planning on similarly making it easier in the terminal to manage and visualize the CIDRs in innernet networks. I'm hoping innernet can become a fun way to learn networking in a safe (and cheap) virtual environment.

I took a CCNA 200-301 class on udemy to learn all that stuff.

https://www.davidc.net/sites/default/subnets/subnets.html

This is the best subnet calculator, since you can split and join subnets visually.

If you have a lot of free time, Computer Networks By Andrew S. Tanenbaum is top notch.

Check this excellent guide by 3COM, Understanding IP Addressing: Everything You Ever Wanted To Know

https://pages.di.unipi.it/ricci/501302.pdf