Route Summarization Calculator — supernet & exact CIDR aggregation
Paste a list of IPv4 networks to get the smallest summary route, how many extra addresses it swallows, the exact minimal CIDR set, a binary alignment view, and OSPF / EIGRP / BGP / Null0 config for Cisco, YAMAHA RTX and Linux.
🔒 Runs entirely in your browser. Your input and output are never sent to a server or stored — fully private.
Exact minimal set
Merges overlapping and adjacent networks into the fewest CIDR blocks that cover exactly the same addresses — nothing more.
Binary alignment
Highlighted bits are shared by every network; the summary prefix length is the number of shared leading bits.
Configuration
Summarize at area or AS boundaries so a flapping subnet inside does not ripple routing updates outward. Always pair a summary with a discard route (Null0 / null / blackhole) on the summarizing router so traffic for unused parts of the block is dropped instead of looping.
About this tool
This route summarization calculator takes a list of IPv4 networks and finds the single smallest summary route that covers them all, then tells you honestly what that costs: how many addresses your networks really contain, how many the summary covers, and how many extra addresses it swallows. Alongside it you get the exact minimal CIDR set — overlapping and adjacent networks merged with nothing added — a binary view that shows where the shared bits end, and ready-to-paste OSPF area range, EIGRP summary-address, BGP aggregate-address and discard-route config for Cisco IOS, YAMAHA RTX and Linux. Everything runs in your browser.
Frequently asked questions
How is a summary route calculated?
Write the networks in binary and count how many leading bits they all share; that count is the summary prefix length, and the shared bits followed by zeros are the summary address. For 172.16.12.0/24 to 172.16.15.0/24 the third octet runs from 00001100 to 00001111, so the first 6 bits of it match: 16 + 6 = 22, giving 172.16.12.0/22. In practice you only need the lowest and highest address in the list, which is what this tool compares, and the binary table shows the result.
Why does my summary cover more addresses than my networks?
Because a CIDR block must be a power of two in size and aligned to that size. 10.1.0.0/24 and 10.1.2.0/24 differ in the second-to-last bit of the third octet, so the smallest block containing both is 10.1.0.0/22 — which also includes 10.1.1.0/24 and 10.1.3.0/24 that you never listed. That summary is only 50% efficient. If those gaps exist elsewhere in your network, the summary will attract their traffic and drop it; the tool flags this and shows the exact set as the safe alternative.
When should I use the single summary and when the exact minimal set?
Use the single summary when the whole block is yours and allocated to the same place — typically an area or site that owns a contiguous range — because one route is the smallest table and hides internal changes best. Use the exact set when efficiency is below 100% and the gaps belong to someone else, when writing prefix lists or firewall objects that must not over-match, or when advertising to the internet, where providers filter prefixes longer than /24 and over-announcing someone else's space is a real incident.
Why do I need a Null0 or discard route with a summary?
To prevent routing loops. If the summarizing router has a default route and receives a packet for an unused part of the summary, without a discard route it would send it back toward the default — and the upstream router, which learned the summary, sends it right back until the TTL expires. A Null0 route for the summary drops it immediately. Cisco IOS installs this automatically for EIGRP summary-address, OSPF area range and BGP aggregate-address; for static summaries, YAMAHA RTX (gateway null) and Linux (blackhole) you add it yourself, which is why the tool generates those lines.
Where can I summarize in OSPF compared with EIGRP and BGP?
OSPF only summarizes at boundaries: an area border router summarizes inter-area routes with area range, and an ASBR summarizes external routes with summary-address. Inside an area every router must hold the same link-state database, so there is nothing to summarize. EIGRP can summarize on any outgoing interface of any router with ip summary-address eigrp. BGP summarizes with aggregate-address, and summary-only suppresses the more-specific routes so only the aggregate is advertised.