CIDR Aggregator
Paste a list of prefixes. It drops the ones already covered, merges true siblings, and leaves the smallest equivalent set.
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What it will and will not merge
Two prefixes merge only when they are genuine siblings: the same length, adjacent, and sharing a parent one bit shorter. 10.0.0.0/25 and 10.0.0.128/25 merge into 10.0.0.0/24.
10.0.1.0/24 and 10.0.2.0/24 do not merge, even though they look adjacent. The smallest prefix covering both is 10.0.0.0/22, which also covers 10.0.0.0/24 and 10.0.3.0/24 — addresses that were not in your list. Silently including them would be wrong, and in a firewall rule or a route filter it would be a security problem.
Prefixes already contained in another are dropped, since listing both adds nothing. IPv4 and IPv6 are aggregated separately and never mixed.
Related tools
IP Range to CIDR converts two addresses into prefixes. Subnet Calculator inspects one prefix in detail.