If you started listening from basic IP addressing which is covered in our list of topics from the beginning and have understood everything, then you won't have any problem understanding subnetting.
I'm going to try and explain subnetting as simply as possible and hope to answer all your questions.
Now, because subnetting is a big topic to talk about, I've split it into a few sections. Logically, as you move on to higher sections, the concepts and material difficulty will increase:
The guide will help you in calculating subnet boundaries and valid host address fast enough for use in a Cisco CCNA or ICND1 exam.
Udemy Course :https://www.udemy.com/ip-adressing-and-subnetting/