This is great.

Quote Originally Posted by Some dude on reddit
If anyone needs help with subnetting and need a different approach from all the YouTube videos and books, think about it this way for a class C subenet:

/24 = 256 possible hosts with 254 available hosts.
/25 = 128 possible hosts with 126 available hosts.
/26 = 64 possible hosts with 62 available hosts.
/27 = 32 possible hosts with 30 available hosts.
/28 = 16 possible hosts with 14 available hosts.
/29 = 8 possible hosts with 6 available hosts.
/30 = 4 possible hosts with 2 available hosts.

Notice that you if start with /24 you have 256 possible hosts. If you go to /25 then all you have to do is divide that by 2 and you'll get 128 possible hosts. If you go to /26 then you divide 128 by 2 and you'll get 64 possible hosts. Don't forget that in order to get the available hosts you need to subtract 2 from the possible hosts.

When it comes to the network mask for class C: /24 is 255.255.255.0

/25 is 255.255.255.128
/26 is 255.255.255.192
/27 is 255.255.255.224
/28 is 255.255.255.240
/29 is 255.255.255.248
/30 is 255.255.255.252


You can start working from /30 with 252 and then subtracting 4 to go /29 with 248, then to go to /28 you subtract 8 to get 240. Notice that as you work your way up, the subtraction doubles. To be honest I memorized it but needed a backup just in case I couldn't remember.