IPv4 Subnet Calculator — CIDR and Host Range

How to use

1. Enter a dotted-decimal IPv4 address, such as 192.168.1.42.
2. Enter its CIDR prefix length as an integer from 0 to 32. Keep the slash and prefix out of the address field.
3. Read the network, mask, broadcast and usable host range.
4. Copy the results into a configuration note or subnet planning document.

Calculation rules

An IPv4 address contains 32 bits. A prefix of /n reserves n bits for the network and leaves 32-n bits for addresses. The total address count is 2^(32-n). The subnet mask has ones in the network positions; the wildcard mask has the opposite bits.

For prefixes from /0 through /30, the network and broadcast addresses are excluded from the usable host count. For /31, both addresses are treated as point-to-point endpoints. For /32, the result is a single host address. These are arithmetic conventions; they do not test support on a particular router or operating system.

Examples

Address and prefix	Network	Usable range
192.168.1.42 /24	192.168.1.0/24	192.168.1.1–192.168.1.254
10.0.0.3 /31	10.0.0.2/31	10.0.0.2–10.0.0.3
203.0.113.7 /32	203.0.113.7/32	One address: 203.0.113.7

Input limits and interpretation

Each octet must be a decimal integer from 0 to 255. Leading-zero forms such as 01.2.3.4, hostnames and IPv6 are rejected to avoid ambiguous interpretation. A /0 covers the entire IPv4 address space mathematically, but its host count is not a count of assignable public addresses.

Before applying a network change, account for your gateway, DHCP allocation, reserved addresses and existing routes. This calculator does not inspect those settings.

The /31 point-to-point convention is described in RFC 3021.