IPv4 CIDR Range Expander

A request lands asking you to allow 10.20.0.0/18 through the firewall, and you need to know right now where that range starts, where it ends and how many addresses it covers. Counting bits in your head works for /24 and /16; at /18, /27 or /30 mistakes creep in fast. Getting the upper bound wrong is the kind of error you discover when someone loses access on a Friday night.

You do not have to type the network address. The tool ANDs the address you entered with the mask, so 192.168.1.77/24 resolves to the network 192.168.1.0/24, which helps when all you have is one host IP and a prefix. Total addresses is 2 to the power of (32 minus n), and usable is that total minus two, with two exceptions: /31, where RFC 3021 puts both addresses to work on point-to-point links, and /32, which stands for a single host. The wildcard shown is the inverted mask, the form Cisco ACLs expect.

Despite the name, the output is a summary of the range rather than an address-by-address list — a /8 would run past 16 million lines. Input must follow a.b.c.d/n: dotted decimal masks, hyphenated ranges and IPv6 are not parsed. Double-check the prefix before pasting a rule, since swapping /16 for /18 changes the block size by a factor of four. All the math happens in your browser.