Route Summarization Calculator
Find the smallest common summary route (CIDR block) that covers a list of individual network addresses.
Inputs
One network per line, in CIDR notation, e.g. 192.168.0.0/24
Summary Route
192.168.0.0/22
Networks Covered
4
Total Addresses in Summary
1,024
Unused (Wasted) Addresses in Summary
0
Step by step
Common prefix length
common_prefix = length of matching leading bits across all input networks
Summary Route
= 192.168.0.0/22
Networks Covered
= 4
Total Addresses in Summary
= 1,024
Unused (Wasted) Addresses in Summary
= 0
How it works
Route summarization finds the shortest common prefix shared by every network in a list, producing a single supernet route that covers them all with one routing table entry instead of many. It is found by comparing the binary representation of each network address bit by bit from the left, and stopping at the first bit position where any two networks disagree — everything before that point is the common prefix. Summarization reduces routing table size and speeds convergence, but if the summarized networks are not perfectly contiguous and aligned, the summary route ends up covering extra address space that isn't actually assigned, shown here as wasted addresses.
Formula
Common prefix length
common_prefix = length of matching leading bits across all input networks
- p_common
- Length of the shared leading bit pattern
Frequently Asked Questions
Why does route summarization sometimes cover addresses that don't exist?
The summary route is defined purely by the longest common bit prefix of the input networks, which produces a block sized to a power of two. If the actual networks aren't contiguous or don't fill that entire power-of-two block, the summary inevitably includes address space beyond what was originally assigned — this is normal and usually an acceptable tradeoff for a smaller routing table.
What's the risk of an imprecise or overly broad summary route?
A summary that covers unassigned address space can cause routers to send traffic for those unused addresses toward the summarizing router, which then has no specific route and must drop it or send an ICMP unreachable — usually harmless, but worth being aware of, especially if that unused space might later be assigned elsewhere.
Does route summarization work well with any arbitrary set of networks?
It works best when networks are allocated contiguously and aligned to natural binary boundaries (e.g. a /22 split evenly into four /24s). Scattered or non-contiguous networks force the common prefix calculation to a much shorter (broader) length, producing a summary that wastes a lot of address space or, in the worst case, is barely smaller than advertising each route individually.