Reading CIDR notation

CIDR (Classless Inter-Domain Routing) writes a block as address/prefix, where the prefix is the number of leading network bits. /24 means the first 24 bits are fixed and the remaining 8 vary — 256 addresses. Each step down the prefix doubles the block: /24 = 256, /23 = 512, /22 = 1024.

block size = 2^(32 − prefix)  ·  a /n block always starts on a multiple of its own size

Aggregation & expansion

Two adjacent, equal-size blocks that share every bit above the split can be summarized (aggregated / supernetted) into one shorter prefix: 10.0.0.0/24 + 10.0.1.0/24 → 10.0.0.0/23. This tool works both ways — collapse a messy IP range into the fewest CIDR blocks, or expand a block into its full address range — which is exactly what you need when writing firewall rules, route summaries, or ACLs that must stay short and correct.

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