CIDR Aggregation / Supernet Finder

Paste in a list of CIDR blocks or addresses and get back the smallest single CIDR block that covers all of them — the calculation behind route summarization, and the thing you actually want when a router's showing forty individual routes for what should be a handful of advertised prefixes.

Why summarization is worth doing, not just theoretically nice

Every route in a routing table costs memory and lookup time, and at scale that adds up. If a site has been allocated ten contiguous /24s and each one gets advertised individually via BGP or OSPF, that's ten entries where one summarized /20 would do the same job — smaller routing tables, faster convergence when something changes, and a routing table that a human can actually read during an incident instead of scrolling past dozens of near-identical entries trying to spot the one that matters.

What "smallest covering supernet" actually means

Given a scattered set of subnets, this finds the smallest CIDR block whose network and broadcast address bracket every address in every block you listed — which is exactly the calculation you'd do by hand to figure out what to actually advertise upstream, or what single firewall rule could replace a list of individual subnet exceptions.

The efficiency number is doing real work

A supernet almost never fits your actual subnets perfectly — there's usually address space inside the summarized block that isn't part of any of your listed subnets, which is the tradeoff you're accepting in exchange for a smaller routing table. The efficiency percentage here tells you how much of the resulting block is "real" versus how much is just there because CIDR blocks have to be power-of-two aligned. Low efficiency isn't necessarily a problem — it might just mean your subnets are scattered further apart than they need to be, which is worth knowing before you request more address space from upstream, since a tighter allocation plan next time would summarize more cleanly.

Where this is actually used

Configuring route summarization on OSPF area border routers or BGP route aggregation, collapsing a list of individual firewall allow-rules into a single range, and sanity-checking whether a proposed address allocation from an ISP is going to summarize well against your existing space before you commit to it — all the same underlying question, just asked from a different angle each time.