What route summarization is for
Every route in a router's table costs memory and CPU to process. If a network advertises four separate /24 subnets that happen to sit next to each other, a neighboring router doesn't need four entries — it can be told about one /22 route that covers all four addresses at once. This is route summarization (also called supernetting or aggregation), and it's a routine part of designing any network with more than a handful of subnets.
The trick is that summarization only works cleanly along power-of-two boundaries. Four /24 networks starting at 10.0.0.0 summarize perfectly into 10.0.0.0/22, because a /22 block is exactly four /24s wide, starting at the right address. Three of those same networks still need the full /22 to summarize into one route — there's no mask that covers exactly three — so the summary ends up advertising a fourth block of address space that isn't actually in use.
Reading the efficiency score
This tool shows what percentage of the resulting summary block is actually covered by the subnets you listed. 100% means the summarization is exact with no waste. Anything lower isn't necessarily wrong — it's the normal cost of aggregating subnets that don't perfectly fill a power-of-two boundary. It's worth knowing about mainly so you don't accidentally assign that "extra" space to something that shouldn't be reachable through the same summarized route.
Frequently asked questions
What is route summarization?
Combining multiple contiguous subnet routes into a single, larger route advertisement — reducing how many entries a router needs to hold and process.
Why can't 3 subnets always summarize as cleanly as 4?
A summary route must start on a valid boundary for its prefix length. Three /24s still need a full /22 to cover them, which means the route also advertises a fourth, unused /24 block.
Is it a problem if a summary route covers unused address space?
Not inherently — it's a normal trade-off. It only matters if that unused space later gets assigned somewhere it shouldn't be reachable from.