Bit counting when subnetting

I was going fair on my Udemy networking course until I go to lesson 47. Subnetting Large Networks Part 2, my brain started smoking. Up to this point I had been dividing the network/hosts on the forth octet. And when counting the bits that I was going to borrow from the host portion of the address I wouldn't even count the bits in the network portion of the address. If it was class C I would only count the bits in the forth octet, if it was a class B I would only count the bits from bit 17 and forward/right, and if it was class A from 9 forward/right. But when I got to this lesson the instructor divides the network on the third octet and for whatever reason he started counting the network bits from the beginning, the very first bit in the first octet. I've attached the file. My network/cicso training has come to a dead stop.