How to Use This Tool
Paste the networks you want to advertise as one route. You get the summary prefix, and — more usefully — the ranges that summary claims which you did not list.
The summary follows the bits, not your list
A summary route is the shortest prefix whose network address matches all of your networks. Which means it is decided entirely by how many leading bits they have in common, and the router has no way to express "these two but not the ones between".
Take 192.168.1.0/24 and 192.168.2.0/24. In binary the third octet is 00000001 and
00000010: they first differ at the seventh bit, so the common prefix is 22 bits long. The
summary is 192.168.0.0/22, and a /22 spans four /24s — .0, .1, .2 and .3.
You listed two. You are now advertising four. If another router owns 192.168.0.0/24 and your summary is more specific than whatever it has, traffic for that network arrives at you and gets dropped. This is the classic summarization outage, and it looks like an intermittent problem because it only affects the prefixes nobody was thinking about.
Three conditions for an exact summary
A set of networks summarizes exactly — no extra addresses — only when all three hold:
- Contiguous. No gaps. A missing /24 in the middle is still covered by the summary.
- Power-of-two count. Three /24s cannot become one prefix; the smallest prefix covering three covers four.
- Aligned to its own size. A block of four /24s must start on a multiple of four: .0, .4, .8. Starting at .1 or .2 forces a shorter prefix that reaches backwards.
Alignment is the one that surprises people, and it is the same rule as in subnetting: a block must begin at a multiple of its own size. 192.168.1.0 through 192.168.4.0 is four contiguous /24s and still does not summarize to a /22, because it does not start on a /22 boundary.
When over-claiming is fine, and when it is not
Advertising more than you own is safe in exactly one situation: you own the extra space too, or nobody does. That is common inside one organisation, where 10.4.0.0/14 covering all of 10.4–10.7 is fine because the whole range is yours whether or not it is in use yet.
It is not safe at a boundary you do not control — to an ISP, to a peer, or across a merger where two networks both use RFC 1918 space. There, switch this tool to the exact-summaries mode: it returns the smallest set of prefixes that covers your networks and nothing else. Two or three routes that are correct beat one route that black-holes a neighbour.
Why summarize at all
Every route consumes memory on every router that carries it, and every route is something to recompute when a link flaps. Summarizing also hides instability: if one /24 inside a summarized block goes up and down, neighbours outside never see the change, because the summary is still valid. That containment is usually worth more than the table size saving.
