Subnetting is used to take a big network and segment it out to make smaller networks. Below is a cheat sheet to easily subnet, you can make this chart during exams from memory.
128 64 32 16 8 4 2 1 – GROUP SIZE (Don’t forget to remove 2 for subnet ID and broadcast address)
/25 /26 /27 /28 /29 /30 /31 /32 – CIDR Notation
2 4 8 16 32 64 128 256 – Number of networks
example, 192.168.100.128 / 25
The first host will be 192.168.100.129
The broadcast address is 192.168.100.255
There are 2 networks, 192.168.100.0/25 and 192.168.100.128/25
Each network will have 126 usable host (.0 and .128 are used as subnet IDs, and the last ip is the broadcast address)