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CIDR Calculator

Test IP membership and summarise multiple CIDR blocks.

Use the CIDR Calculator tool

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What this cidr calculator does

CIDR Calculator works in both directions. In membership mode it tells you whether an address belongs to a given prefix, which is useful for verifying ACLs and routing rules. In aggregation mode it takes a list of CIDR blocks and merges adjacent blocks into the shortest possible summary — the same idea used by route summarisation in BGP and OSPF.

All calculations are performed locally with 32-bit or 128-bit integer arithmetic. The output shows both the input blocks and the resulting summaries, so you can see exactly what was collapsed.

Why use it

  • Confirm that a firewall rule actually covers the host you expect.
  • Summarise dozens of cloud provider ranges into a compact ACL.
  • Validate that two networks can be aggregated without accidentally including addresses you do not own.

How to use it

  1. 1Choose Membership mode to test an IP against one or more prefixes.
  2. 2Or choose Aggregate mode and paste one CIDR block per line.
  3. 3Press Calculate to see range details or merged summaries.
  4. 4Review warnings when a merge would include address space outside the input list.

Worked examples

Membership

Input

IP 10.20.30.40 · CIDR 10.20.0.0/16

Output

Inside · range 10.20.0.0 – 10.20.255.255

Aggregation

Input

192.168.0.0/25
192.168.0.128/25

Output

192.168.0.0/24

The two halves of the /24 combine exactly, so the summary covers no extra addresses.

Common mistakes

  • Aggregating blocks that are not adjacent, which silently includes addresses outside your allocation.
  • Assuming 0.0.0.0/0 is harmless — it matches every address, including traffic you did not intend to allow.
  • Mixing IPv4 and IPv6 in one aggregation request; aggregate each family separately.

Frequently asked questions

What does CIDR mean?
Classless Inter-Domain Routing. The /n suffix states how many leading bits form the network prefix, replacing the old classful A/B/C boundaries.
Can this tool aggregate IPv6 prefixes?
Yes. Prefixes are aggregated within the same address family using 128-bit arithmetic.
Does aggregation change what I can reach?
If the summary covers only addresses from your inputs, no. If blocks are not adjacent, the summary necessarily includes the gap between them — always review the highlighted output.

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