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About CIDR Summarization Calculator – IPv4 Route Aggregation
Aggregate multiple IPv4 CIDR networks into the smallest mathematically exact set of summarized routing prefixes.
Automatically eliminates duplicate subnets, strips redundant contained blocks, and performs deterministic sibling-pair merges without over-summarizing.
Input is processed locally in your browser and is not sent to Navorika.
How the calculation works
Network normalization
Each input CIDR is normalized to its network boundary: Network = IP & Mask.
Contained subnet pruning
Subnets fully enveloped by a larger supplied parent network are eliminated as redundant.
Exact sibling aggregation
Two adjacent equal-sized blocks of prefix P merge into prefix P-1 if and only if their union exactly equals the parent block without covering unsupplied addresses.
How to use this tool
- 1Paste or enter one or more IPv4 CIDR blocks (one per line, comma, or space-separated).
- 2The calculator normalizes all inputs to strict network boundaries.
- 3Redundant subnets and duplicate entries are removed.
- 4Adjacent sibling subnets are iteratively aggregated into parent prefixes.
- 5Copy the concise summarized CIDR list for BGP, OSPF, or firewall configuration.
Understanding the result
Two sibling /25 subnets (e.g. 192.168.0.0/25 and 192.168.0.128/25) combine into a single /24 supernet (192.168.0.0/24).
Non-adjacent or misaligned blocks cannot be merged because doing so would advertise address space not provided in the original input.
Route summarization reduces routing table size and memory overhead on core routers.
Important limitations
- IPv4 only. IPv6 route summarization follows different hierarchical delegation boundaries.
- The calculator strictly forbids loose summarization that would encompass unsupplied address ranges.
- Operational routing decisions must also take path metrics, AS boundaries, and policy filtering into account.
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Frequently asked questions
Why cannot 192.168.0.0/25 and 192.168.1.0/25 be summarized into /24?
192.168.0.0/25 needs sibling 192.168.0.128/25 to complete the 192.168.0.0/24 block. Merging with 192.168.1.0/25 would leave holes and improperly claim unsupplied addresses.
What is a sibling network in CIDR?
Two networks with identical prefix length P whose addresses differ only at the (32 - P)th bit. Together, their exact union constitutes the parent prefix P - 1.
How does route summarization benefit networks?
Summarization shrinks router routing tables, conserves memory and CPU cycles during route recalculations, and confines routing instability (flapping) to local network segments.
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