Important Group Size Calculation

Practical Networking’s Subnetting Mastery playlist

---

How to Calculate group size for the 3rd Octet and below (/1 - /24) in the Cheat Sheet

The Solution

From the cheat sheet, we know that every CIDR notation has its own group size which indicates how many hosts it can hold, but, in the cheat sheet table, the group size numbers only indicate the amount of hosts that can be held within /25 till /32. It doesn’t mention the group sizes for /1 till /24.

However, it turns out you can just continue counting the group size by DOUBLING the previous CIDR notation’s group size.

• So, from our current cheat sheet, we know that /25 has group size 128.
• To get /24’s group size, you double the previous CIDR’s group size, and you get 256.
• To get /23’s group size you double the previous CIDR’s group size, and you get 512.
• To get /22’s group size you double the previous CIDR’s group size, and you get 1024.
• To get /21’s group size you double the previous CIDR’s group size, and you get 2048.
• You can continue doing this process until you get to the CIDR notation you want.

And that’s how you get the group size values for all the CIDR notation in the list.