Route Summarization Calculator
Find the supernet that covers a run of contiguous subnets.
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Route Summarization Calculator by TheCalcTools
How the Route Summarization Calculator works
Enter the first network and the common prefix, then how many networks follow it contiguously.
The summary prefix is the prefix minus ceil(log₂ count).
The start address must be a multiple of the summary block, otherwise one route cannot cover it exactly.
The formula
Worked example
Using the default values shown in the calculator:
| Octet 1 | 10 |
|---|---|
| Octet 2 | 0 |
| Octet 3 | 8 |
| Octet 4 | 0 |
| Prefix of each network (/n) | 24 |
| Number of contiguous networks | 6 |
- Six /24 networks from 10.0.8.0 need a block of 2,048 addresses.
- Summary route: 10.0.8.0/21. Waste: 512 addresses. Start is aligned to the summary boundary.
Result:
| Summary route | 10.0.8.0/21 |
|---|---|
| Addresses covered by the summary | 2,048 |
| Addresses actually in your networks | 1,536 |
| Extra addresses in the summary | 512 |
| Alignment | Start is aligned to the summary boundary |
Frequently asked questions
Why does summarising 6 networks create extra addresses?
A summary must be a power of two. Six /24s need an 8-block /21, so two /24s of address space are covered but not used.
Is over-summarisation dangerous?
It can attract traffic for addresses you do not own, so check that the extra space is not routed elsewhere.
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.