Route Summarization Calculator

Route summarization, also called CIDR aggregation or route aggregation, is the process of combining multiple specific IP prefixes into a single, less-specific summary prefix that covers all of them. For example, 10.0.0.0/24 and 10.0.1.0/24 can be summarized as 10.0.0.0/23. This reduces routing table size, improves convergence, and simplifies BGP prefix-list and route-map filters.

Use summarization when advertising customer prefixes to peers (to avoid leaking specifics), when writing inbound prefix-list filters (to cover a range of customer routes with fewer entries), and when configuring aggregate-address in BGP. Over-summarization can cause black holes if the summary covers addresses not actually reachable — always verify coverage after summarizing.

A supernet calculator finds a single covering summary for two prefixes. This tool handles any number of prefixes and finds the minimal set of summaries — meaning it will produce multiple summary routes if the input cannot be fully collapsed into one. It also shows the step-by-step merge operations so you can understand exactly how the aggregation happened.

CIDR aggregation merges adjacent same-size blocks, which may include addresses not in the original set. For example, 10.0.0.0/25 and 10.0.1.0/25 cannot be merged (they are not adjacent /25 blocks in the same /24), but 10.0.0.0/25 and 10.0.0.128/25 can merge to 10.0.0.0/24. The extra addresses metric shows how many additional IPs are covered by the summary. For BGP filters, this is usually acceptable.

Use 'aggregate-address <prefix> <mask> summary-only' to advertise only the summary and suppress the specifics. Or add the summary as a static route pointing to Null0 and redistribute it. For inbound filtering, use prefix-list entries with 'ge' and 'le' keywords to match a range: 'ip prefix-list FILTER seq 5 permit 10.0.0.0/23 le 24' matches all /24s within 10.0.0.0/23.

The algorithm works in rounds: (1) remove any prefix already covered by a less-specific prefix in the list, (2) sort remaining prefixes by length and network address, (3) find adjacent pairs — two /n prefixes whose network addresses differ only in the last bit of the /n mask — and merge them into a /(n-1), (4) repeat until no more merges are possible. The result is the minimal set of prefixes that covers the entire input address space.