CIDR / subnet calculator

How to use the CIDR calculator

1. Type or paste an IPv4 CIDR block into the CIDR block field, for example 10.0.0.0/8 or 192.168.1.0/24.
2. If you have a host address rather than the network address, enter it with its prefix, for example 192.168.1.130/26. The tool masks it to the network, 192.168.1.128.
3. Read the results table: network and broadcast address, subnet mask, wildcard mask, first and last usable host, usable hosts, total addresses and IP class.
4. If a message appears instead of results, fix the input: four numbers from 0 to 255 without leading zeros, a slash, then a prefix from 0 to 32.
5. Select a value in the table and copy it into your router, firewall or cloud network settings.

Common uses

• Planning subnets for a VPC or office network and checking that each block has enough usable hosts
• Finding the range a host address belongs to: 10.1.4.77/22 sits in 10.1.4.0 to 10.1.7.255
• Reading a subnet mask bit by bit after converting each octet to binary
• Setting up a new Linux server: plan its address here, then work out the permission bits for its config files

Technical specification

• Algorithm or formula: Bitwise math on a 32-bit unsigned integer view of the IP. Network = ip & mask, broadcast = network | ~mask, total hosts = 2^(32 − prefix), usable hosts = total − 2 for prefixes 0 to 30.
• Browser API or library: Pure JavaScript bitwise operations. No external library.
• Input limits: IPv4 CIDR notation only (e.g. 10.0.0.0/8 to 192.168.1.5/32).
• Output: Network and broadcast addresses, subnet mask and wildcard mask (dotted decimal), first and last host, total host count, and IP class designation (A, B, C, D, E).
• Known limitations: IPv4 only; IPv6 addresses are not accepted. Variable-length subnet masking (VLSM) helper output is not included.

Frequently asked questions

What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation expresses an IP address and its network prefix length together, for example, 192.168.1.0/24, where /24 means the first 24 bits are the network identifier.

Is my data sent to a server?

All calculations are pure JavaScript bit arithmetic running in your browser. Our servers are not involved at any point.

How many usable hosts are in a /24 network?

A /24 network contains 256 addresses. Subtract 2 (network and broadcast addresses) for 254 usable host addresses.

Does the tool support IPv6?

The calculator handles IPv4 CIDR blocks from /0 to /32. An address that contains a colon, as every IPv6 address does, shows a message asking for an IPv4 block instead.