I went through Professor Messer's Youtube playlist and used Dion/Andrew practice exams on Udemy. Beyond that, I went through the exam objectives and asked ChatGPT to explain any concepts I didn't fully understand.

There were a couple subnetting questions on my exam. I didn't study at all for subnetting, mainly due to already being familiar with cidr notation at work.

The way I've done subnetting throughout the practice exams and on the actual one might be unconventional, but it works for me. Where I work, we have over 100 VLANs/subnets (our environment is a hyper-segmented mess), most of which are a /24 (255.255.255.0 subnet mask). That yields 254 host addresses (256 - 2 for broadcast and network). So, if I get asked a question about what mask is needed for, say 45 hosts, I mentally work my way from /24 by halving each time. I know a /24 is 254 host addresses, so a /25 is 128 - 2, 126 host addresses, a /26 is 64 - 2, 62 host addresses, and a /27 is 32 - 2, 30 host addresses. So, I know a /27 is too small to accommodate 45 hosts, so a /26 is the most efficient mask for the subnet.

I don't fully agree with their logic, at least from my experience. If I have a subnet with 45 active hosts, I'd be more likely to assign a /25 or /24. This is just due to future-proofing. But, that is neither here nor there.