CIDR Aggregator / Supernet Calculator
Combine a list of IPv4 CIDR blocks or plain addresses into the smallest set of aggregated (summarised) CIDR blocks. Merges overlapping and adjacent ranges — all in your browser.
10.0.0.5) counts as a /32.
What CIDR aggregation is for
CIDR aggregation — also called supernetting or route summarisation — takes a scattered list of address blocks and collapses them into the fewest possible CIDR blocks that describe exactly the same addresses. Routers do this to shrink routing tables; engineers do it to write cleaner firewall rules and prefix lists. Fewer, tighter blocks mean less to maintain and fewer chances for a typo to open a hole.
The algorithm has two stages. First, every entry is converted to a start–end integer interval and the intervals are sorted and merged wherever they overlap or sit directly next to each other. Second, each merged interval is re-expressed as the minimal set of aligned CIDR blocks — the same technique used to turn an arbitrary IP range into CIDRs. The result is a before/after count so you can see how much was saved.
Two equal-sized blocks only fuse into one bigger block when they are adjacent and the lower one is aligned to the larger boundary. 192.168.0.0/24 + 192.168.1.0/24 → 192.168.0.0/23, but 192.168.1.0/24 + 192.168.2.0/24 cannot become a single /23 because 192.168.1.0 is not on a /23 boundary. The tool still summarises such pairs as tightly as the math allows.
If you are still building intuition for how a prefix like /23 relates to its address count and mask, the guide on what a subnet mask is is a good companion, and reserved IP addresses explains which blocks you are likely aggregating on a private network. Once you have your summarised list, verify coverage with the IP in Subnet Checker or derive the ACL form with the Wildcard Mask Calculator.
All parsing and math run locally in JavaScript. Nothing you paste is uploaded, so internal addressing plans stay on your machine.
Frequently asked questions
What does CIDR aggregation do?
CIDR aggregation, also called supernetting or route summarisation, merges several IP blocks that touch or overlap into the fewest CIDR blocks that cover exactly the same addresses. For example 192.168.0.0/24 and 192.168.1.0/24 combine into a single 192.168.0.0/23.
Can two adjacent subnets always be combined?
Two blocks of the same size only merge into one larger block when they are adjacent and the lower block is aligned to the larger block’s boundary. 192.168.0.0/24 and 192.168.1.0/24 merge into 192.168.0.0/23, but 192.168.1.0/24 and 192.168.2.0/24 do not. This tool still summarises them as tightly as possible.
What input formats are accepted?
Enter one entry per line. Each line can be a CIDR block such as 10.0.0.0/24 or a plain IPv4 address such as 10.0.0.5, which is treated as a single-address /32. Blank lines are ignored and invalid lines are reported.
Is the aggregation lossless?
Yes. The output covers exactly the same set of addresses as the input, never more and never fewer. Overlapping and adjacent ranges are merged, then each merged range is expressed as the minimal list of aligned CIDR blocks.
Want the theory? Read the guides → · Visit the zoo →