What is the difference between the exact cover and a supernet?

The exact cover is the smallest set of prefixes that covers exactly the addresses you listed, adding nothing. A supernet is one prefix that contains them all, so it usually also advertises addresses in between that belong to somebody else.

Why can 10.0.0.0/24 and 10.0.2.0/24 not be summarized to 10.0.0.0/22?

It can be expressed that way, but the /22 is 10.0.0.0 through 10.0.3.255, which also covers 10.0.1.0/24 and 10.0.3.0/24. Those two blocks are holes: announcing the /22 attracts their traffic to a router with no route for it.

What are holes and why are they dangerous?

A hole is address space inside the aggregate that no more-specific route claims. If the aggregate is announced with summary-only, the more-specifics are withdrawn and the hole is advertised as reachable, so traffic for it arrives and is dropped — a silent outage that only appears when someone uses an address in the hole.

When is the max-prefixes mode not optimal?

It only merges direct siblings, which is the only merge that adds no address space. A set with no sibling pair left stays over budget rather than being widened, so you may end up with more prefixes than you asked for; the tool says so instead of inventing address space.

Route Summarization Calculator

Reduce a list of prefixes to the minimal exact cover, the single covering supernet, and the holes that the supernet would advertise but nobody owns.

Input

One CIDR block per line

Blank lines and lines starting with #, ! or // are ignored.

Output

Export

2 prefix(es) in the cover

Advertises 1,024 addresses that are not in this list

The smallest single prefix covering everything is 10.0.0.0/21. Announcing it would attract traffic for the address space below, which no route here owns:

10.0.2.0/23 · 512 addr
10.0.6.0/23 · 512 addr

Announcing this aggregate blackholes every address listed above unless the space genuinely belongs to you. Prefer the exact cover, or add the missing blocks.

Summary

Minimal exact cover — no address space is added

Input blocks: 4
Cover prefixes: 2
Cover prefixes: 2
Extra addresses: 0
Smallest supernet: 10.0.0.0/21
Holes vs supernet: 2
Status: Exact

Exact coverMode: exact2 hole block(s)

Exact minimal cover

Every address in this list belongs to the input

Prefix	Network	Broadcast	Addresses
10.0.0.0/23	10.0.0.0	10.0.1.255	512
10.0.4.0/23	10.0.4.0	10.0.5.255	512

Selected cover

Identical to the exact cover

Prefix	Addresses	Owned?
10.0.0.0/23	512	Exact
10.0.4.0/23	512	Exact

Holes — space the supernet would advertise

1,024 addresses nobody in this list owns

10.0.2.0/23
10.0.6.0/23

Cover as a prefix list

Paste-ready lines for an ACL or route filter

Exact cover

10.0.0.0/23
10.0.4.0/23

Which mode to use

Exact cover is safe: it announces only the address space you listed. Single supernet is the smallest one-prefix summary, and it silently advertises everything between your blocks — read the hole list above before you use it.

Max prefixes reaches your budget by promoting one block at a time to a wider prefix, always choosing the promotion that invents the fewest addresses. A promotion can cover a sibling or the larger half of the parent block, which is how two blocks that are not direct siblings still merge. The extra-space ceiling caps how far that goes: when the budget cannot be reached without advertising more than the ceiling allows, the result stays short of the budget and the note above says by how much. The greedy is deterministic but not provably minimal — a different set of promotions can sometimes reach the same budget with less invented space, so check the extra-address figure before you publish the plan.

Frequently asked

What is the difference between the exact cover and a supernet?: The exact cover is the smallest set of prefixes that covers exactly the addresses you listed, adding nothing. A supernet is one prefix that contains them all, so it usually also advertises addresses in between that belong to somebody else.
Why can 10.0.0.0/24 and 10.0.2.0/24 not be summarized to 10.0.0.0/22?: It can be expressed that way, but the /22 is 10.0.0.0 through 10.0.3.255, which also covers 10.0.1.0/24 and 10.0.3.0/24. Those two blocks are holes: announcing the /22 attracts their traffic to a router with no route for it.
What are holes and why are they dangerous?: A hole is address space inside the aggregate that no more-specific route claims. If the aggregate is announced with summary-only, the more-specifics are withdrawn and the hole is advertised as reachable, so traffic for it arrives and is dropped — a silent outage that only appears when someone uses an address in the hole.
When is the max-prefixes mode not optimal?: It only merges direct siblings, which is the only merge that adds no address space. A set with no sibling pair left stays over budget rather than being widened, so you may end up with more prefixes than you asked for; the tool says so instead of inventing address space.

Frequently asked

Next