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The Block Size Trick

Lesson 24 of 46 in Coddy's IP Addressing and Subnetting course.

Here is the trick that turns subnetting from binary arithmetic into mental math: 256 minus the interesting mask octet = the block size, and subnets start at multiples of that block.

Mask 255.255.255.192 (/26): 256 - 192 = 64. So in the last octet, subnets begin at 0, 64, 128 and 192. Mask ending in 240 (/28): 256 - 240 = 16, so boundaries land every 16.

To place any address, you no longer AND bits: you just ask which multiple of the block size it falls on. The next lessons drill exactly that.

Challenge

Beginner

For a /26 mask, print all the last-octet subnet boundaries on one line, separated by single spaces, lowest first.

Expected output:

0 64 128 192

Try it yourself

Terminal

All lessons in IP Addressing and Subnetting

1Bits and Binary

Numbers as Bits Binary to Decimal Powers of Two Recap - Binary Warm-up

4CIDR Notation

Goodbye Classes Reading CIDR Block Sizes Recap - CIDR Reading

7Special Addresses

Private Ranges Loopback Link-Local Zero and Broadcast

10Capstone - The Office

The Floor Plan Sizing Each Team Carving the Block The Router's View Recap - Ship the Plan

2IPv4 Anatomy

Four Octets The 32-Bit View Network and Host Valid or Not Recap - Read an Address

5Inside a Subnet

Network Address Broadcast Address Usable Hosts First and Last Host Recap - Subnet Facts

8Planning Networks

Right-Sizing VLSM Step by Step Allocation Order Checking for Overlap Recap - Plan a Network

3The Subnet Mask

What a Mask Does Mask to Prefix Prefix to Mask ANDing It Out Recap - Mask Basics

6Subnetting Drills

The Block Size TrickWhich Subnet Am I In Splitting a /24 Splitting Again Recap - Subnetting Drills

9IPv6 First Steps

Why IPv6 Reading IPv6 Prefixes in IPv6 Recap - IPv6 Basics

Practice on your own: Terminal playground