Route Summarisation Calculator

Work out route summarisation instantly with clear inputs, formula shown and shareable results.

About Route Summarisation Calculator

Summarising two networks means finding how many high-order bits they share. XOR the two addresses and count the leading zeros: that count is the longest prefix that still contains both, and applying it to the lower address gives the summary network. Fewer common bits means a shorter, broader summary.

Formula

Common prefix length

matching bits = count of leading zeros in (A XOR B); summary = (lower address AND mask) / matching bits

Worked example

With the default values below, Route Summarisation Calculator returns Summary route of 172.16.4.0/22. Change any input above and every figure here is recalculated in your browser.

Inputs used

First network: 172.16.4.0
Last network: 172.16.7.0

Results

Summary route: 172.16.4.0/22
Common leading bits: 22 bits
Addresses covered: 1,024
Summary mask: 255.255.252.0

Step by step

1Values used
2Common prefix length
3Summary route= 172.16.4.0/22
4Common leading bits= 22 bits
5Addresses covered= 1,024
6Summary mask= 255.255.252.0

Assumptions and limits

The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is my summary broader than expected?

A single bit difference high in the address forces a short prefix. Networks such as 10.1.0.0 and 10.254.0.0 only share eight bits, so the only summary containing both is 10.0.0.0/8.

Does the summary cover networks in between?

Yes, and that is the point of summarisation — but it also covers everything else in that prefix, so only summarise ranges you own end to end.