This route summarization calculator merges a list of networks into the fewest routes that cover exactly those addresses, and finds the single summary route that covers them all. It runs entirely in your browser: nothing you type is sent to our server.
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Table of Contents
Exact merge or single summary
Two answers are shown. The exact summary combines adjacent and overlapping networks without adding any address you did not list: 10.1.0.0/24 to 10.1.5.0/24 becomes 10.1.0.0/22 and 10.1.4.0/23. The single summary route is the shortest prefix that contains everything, here 10.1.0.0/21, which also covers 10.1.6.0-10.1.7.255 (512 extra addresses).
When a summary is safe
- Advertise a summary only if you own the whole block. Otherwise you attract traffic for addresses that are not yours.
- Install a discard route (to Null0) for the summary so packets to unused parts are dropped instead of looping.
- In OSPF, summaries are made at area borders (
area 0 range); in BGP withaggregate-address ... summary-only.
For details of a single block, use the CIDR to IP range converter.
How we checked it
We compared the address library behind this tool with Python’s ipaddress module on 1,105 random cases, including merging networks (collapse_addresses), with no differences, and checked the sample (10.1.0.0/24 to 10.1.5.0/24) by hand.
Frequently asked questions
How do I summarize routes?
Find the longest prefix that all networks share. The calculator does this and also gives the exact set of prefixes that covers only your networks.
What is the difference between summarization and aggregation?
They are used interchangeably: replacing several specific routes with fewer, shorter prefixes. In BGP the command is aggregate-address.
Why should I add a Null0 route for a summary?
Without it, packets to unused parts of the summary follow the default route and can loop between routers.