Subnetting trick: the magic number

Step 1: find the interesting part

Every prefix length has a subnet mask. A /26 means the first 26 bits are the network part, which written in decimal is 255.255.255.192.

Look for the one part of the mask that isn't 255 or 0. That's the interesting octet, where the network part ends and the host part begins. In 255.255.255.192 it's the last part, 192.

Step 2: subtract it from 256

256 - 192 = 64. That 64 is the magic number, also called the block size: every subnet made with this mask is exactly 64 addresses wide.

Why 256? Each octet can hold 256 values (0 to 255). The mask takes some of them for the network and leaves the rest as the block.

Step 3: count up by the magic number

Start at 0 and keep adding 64: 0, 64, 128, 192. Each of those is where a subnet starts in that octet. With 192.168.1.0/26 that gives four subnets:

• 192.168.1.0 (addresses .0 to .63)
• 192.168.1.64 (.64 to .127)
• 192.168.1.128 (.128 to .191)
• 192.168.1.192 (.192 to .255)

Step 4: place an address

Which subnet is 192.168.1.100 in? Look at the interesting octet, 100. It falls between 64 and 128, so it lives in the subnet that starts at 64: 192.168.1.64/26, which runs from .64 to .127.

Try it yourself

Try /27, whose mask is 255.255.255.224. The magic number is 256 - 224 = 32, so subnets start at 0, 32, 64, 96, 128, 160, 192 and 224. Which one holds 10.0.0.77? (It's between 64 and 96, so the subnet starting at 10.0.0.64.) Check your answer with any online IPv4 subnet calculator.

Good to know

• The trick works on the "interesting" part of the mask, the one that isn't 255 or 0. The block size is 256 minus that value.
• 192.168.1.100/26 is in subnet 192.168.1.64 (range .64 to .127).
• Masks that end exactly on a dot (/8, /16, /24) have no interesting part: the block is a whole octet.
• When the interesting part is in the second or third octet, the same trick works there; you just count up in that octet instead.

For CCNA learners

This lines up with CCNA 200-301 exam topic 1.6, configure and verify IPv4 addressing and subnetting: the block-size (magic number) method lets you find subnet boundaries quickly under exam time pressure, without writing out binary.

Answer to the comment question

The magic number for /27 is 32. Its mask is 255.255.255.224, the interesting part is 224, and 256 - 224 = 32. So every /27 block is 32 addresses wide, and the subnets start at 0, 32, 64, 96 and so on.