What the calculator gives you
From an address and a prefix it works out the network address, the broadcast address, the first and last usable host, the total and usable host counts, and the mask in both dotted and wildcard form. It also goes the other way: an arbitrary start and end address become the minimum set of CIDR blocks that covers them exactly, and a block expands into a plain list of addresses.
The prefix is simply how many leading bits are fixed. A /24 fixes 24 bits and leaves 8 for hosts — 256 addresses, of which 254 are usable once the network and broadcast addresses are taken out. Each step down the prefix doubles the block: /23 is 512, /22 is 1024. The two exceptions worth knowing are /31, which is used for point-to-point links where both addresses are usable, and /32, which is a single host.
Reference
When you need it
Writing a firewall rule, an ACL or a whitelist and needing the exact block rather than an approximate one.
Whitelisting a proxy provider's range for IP authentication.
Turning a range a supplier quoted as "from … to …" into CIDR notation your configuration file will accept.
Planning subnets and checking that two of them do not overlap.
Limits
The calculator handles IPv4 and runs entirely in your browser. Expanding a block into a list of addresses is capped, because a /8 is sixteen million lines and the browser, not the server, would be the one to fall over. To find out who a particular address belongs to, use the IP lookup.
Frequently asked questions
What does /24 mean?
That the first 24 bits of the address are fixed and the remaining 8 identify hosts: 256 addresses in total, 254 usable after the network and broadcast addresses are set aside.
Why are two addresses unusable in most subnets?
The first is the network address, which names the subnet itself, and the last is the broadcast address. Neither can be assigned to a host. The exceptions are /31, where both are usable on point-to-point links, and /32, which is one host.
What is a wildcard mask?
The bitwise inverse of the subnet mask — /24 gives 0.0.0.255. Cisco ACLs and several other systems expect it in that form, which is why the calculator shows both.
Can I convert an arbitrary range to CIDR?
Yes. A range that does not align to a single block is expressed as the smallest set of blocks that covers it exactly, which is what firewalls and route tables need.
Does it support IPv6?
Not yet — this calculator is IPv4 only. IPv6 subnetting follows the same prefix logic but with 128 bits, where the host portion is normally so large that host counts stop being a practical concern.
Why is the address list limited?
Because expanding a large block means rendering millions of lines in your browser. The cap keeps the page responsive; if you need a huge enumeration, generate it in a script rather than a page.