Input
Result
The result will appear here.Planning a VPC, writing a firewall rule or reading a cloud security group means turning 10.24.8.77/21 into something concrete: which network it is, how many hosts fit, where the range starts and ends. Enter a block and this calculator gives the network and broadcast addresses, the first and last usable host, the netmask and wildcard mask, and the kind of range it belongs to — private, loopback, carrier-grade NAT and so on. It does the same for IPv6, where a /64 holds 18 quintillion addresses and is still counted exactly.
How it works
- Accepts 192.168.1.10/24, a dotted mask such as 10.1.2.3 255.255.0.0, an IPv6 block such as 2001:db8::/48, or a bare address, which is treated as /32 or /128.
- If the address has host bits set, the block is normalised to its network address, and hostBitsSet in the result says so.
- The check field answers whether an address or a smaller block lies inside; the split field divides the block into longer prefixes and lists the first 256 of them.
- Arithmetic is done on BigInt, and ipaddr.js parses the addresses and names the range type, so large IPv6 blocks are exact rather than rounded.
Where your data goes
Nowhere. This tool runs entirely in your browser: the text you paste is processed by the page and is never transmitted to a server or written to a log.
This tool is free and needs no account. Its results exist only in your open page and are not saved anywhere.
What it costs
This tool is free, with no sign-in and no points.
Common questions
- Why are there two fewer usable hosts than addresses?
- In an IPv4 subnet the first address identifies the network and the last is the broadcast address, so neither can be given to a host: a /24 has 256 addresses and 254 usable hosts. The exceptions are /31, which RFC 3021 allows for point-to-point links with both addresses usable, and /32, a single host. Cloud providers often reserve a few more — AWS keeps five per subnet.
- Why does IPv6 have no broadcast address?
- IPv6 has no broadcast at all; it uses multicast instead. So every address in an IPv6 block is counted, and the last address is simply the last host.
- What is the wildcard mask for?
- It is the netmask inverted — 0.0.0.255 for a /24 — and Cisco access lists and OSPF configuration are written with it instead of the netmask.
- How do I split a /16 into /24 subnets?
- Enter the /16 and type 24 in the split field. The result lists each /24 with its first and last address and gives the total count — 256 in this case. For very large splits, such as a /8 into /30s, the count is exact but only the first 256 subnets are listed.
The open-source behind it
This tool runs on whitequark/ipaddr.js, released under MIT. If you need the same behaviour inside your own program, that is the library to reach for.
whitequark/ipaddr.jsAlso known as
- cidr calculator
- subnet calculator
- ipv4 subnet calculator
- ipv6 subnet calculator
- cidr to ip range
- subnet mask calculator
- split cidr into subnets