Use CIDR Toolkit for firewall rules, cloud security groups, VPN routes, proxy allowlists, and IP access reviews. It calculates exact IPv4 ranges, merges overlapping blocks, subtracts excluded networks, and catches accidental overlap before a rule reaches production.
This is intentionally more than a subnet calculator: paste real allowlists, remove a partner range, collapse redundant entries, or compare two lists from different systems without exposing private infrastructure data.
Analyze mode reports the network address, broadcast address, subnet mask, first and last usable addresses, and host counts for one IPv4 block. List modes accept one CIDR per line to merge adjacent ranges, subtract excluded blocks, or show the exact address range shared by two lists.
The toolkit handles IPv4 CIDR notation. A broader prefix can expose far more addresses than intended, and merging or subtracting networks may change the number of firewall entries. Review copied output against the conventions of your cloud provider, router, or operating system.
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