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Networking Fundamentals · Foundational

Block Size

The increment between consecutive subnet addresses in the interesting octet, equal to 256 minus the mask value in that octet.

Definition

Block size (sometimes called the magic number) lets you list subnet boundaries quickly: with mask 255.255.255.224 the block size is 32, so subnets start at 0, 32, 64, 96 and so on. The broadcast address of each subnet is one less than the next subnet's start.

Example

Host 172.16.5.100/28 has block size 16, so it is in subnet 172.16.5.96 with broadcast 172.16.5.111.

Exam tip

Find the block size first; every other subnetting answer follows from it.

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