Lesson 16: Practical Subnetting: Calculating /25, /26, and /27

Let's assume we start with the network 192.168.1.0/24 (Class C).

Calculation Method: The 'Magic Number' (Block Size)

To find the block size (how often the network addresses increment), subtract the last octet of the custom mask from 256.

Example 1: Subnetting to /25

• Default mask: 255.255.255.0. New mask: 255.255.255.128.
• We borrowed N=1 bit. $2^1 = 2$ subnets.
• Remaining host bits H=7. $2^7 - 2 = 126$ usable hosts.
• Magic Number: $256 - 128 = 128$.

Example 2: Subnetting to /26

• New mask: 255.255.255.192. We borrowed N=2 bits. $2^2 = 4$ subnets.
• Magic Number: $256 - 192 = 64$.

Subnets increment by 64: 0, 64, 128, 192.