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Route Summarization & Supernet Calculator (CIDR Aggregation)

Find the summary route (supernet) that covers a list of IPv4 networks, its mask and wildcard, how many extra addresses it includes, and the smallest exact set of aggregated CIDR blocks.

Networks to summarize

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How it works

write every network as a range of 32-bit numbers · summary prefix = number of leading bits shared by the lowest and highest address · summary = lowest address with the remaining bits zeroed · exact aggregate: merge overlapping or adjacent ranges, then cut each range into the largest aligned power-of-two blocks

Route summarization (supernetting, CIDR aggregation — RFC 4632) advertises one shorter prefix in place of many longer ones, shrinking routing tables and hiding flaps of individual subnets. The single summary route is the longest prefix whose block contains every listed network; if the networks are not contiguous or not aligned, it also covers addresses you did not list, which can black-hole or misroute traffic. The table shows the smallest set of CIDR blocks that covers exactly the listed space, which is what to advertise when an over-broad summary is not acceptable.

Worked example

Summarizing 172.16.0.0/24, 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/24: the first 22 bits are shared, so the summary route is 172.16.0.0/22 with mask 255.255.252.0 and wildcard 0.0.3.255. It covers 1,024 addresses, exactly the four /24s, so the summary is exact.

Sources: RFC 4632 — Classless Inter-domain Routing (CIDR): aggregation · Wikipedia — Supernetwork

Frequently asked questions

How do you calculate a summary route?

Write the lowest and highest network addresses in binary and count the leading bits they share. That count is the summary prefix; zero the remaining bits of the lowest address to get the summary network.

What is the difference between summarization and supernetting?

They are the same idea. "Supernet" usually means combining classful networks into a shorter prefix; "route summarization" is the routing-protocol feature that advertises it.

Why does my summary include extra addresses?

The listed networks are not a contiguous, aligned power-of-two block. For example 10.1.1.0/24 and 10.1.2.0/24 summarize to 10.1.0.0/22, which also covers 10.1.0.0/24 and 10.1.3.0/24.

Where is route summarization configured?

On area border routers in OSPF (area range), on any interface in EIGRP (ip summary-address), and with aggregate-address in BGP.

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This calculator is for educational purposes. Double-check important results before acting on them.