SubnetCalc
Calculate subnet

RFC 1918 reference

Private IPv4 address ranges

Check whether an address belongs to the three RFC 1918 private blocks and calculate the subnet without a lookup or network request.

Private browser calculation

Address, prefix, and host sizing

Stays on this page

Calculation mode

IPv4 address and prefix

Use a separate prefix or mask, or enter address/prefix and leave the second field empty.

A /31 result is blocked until you explicitly confirm its two endpoint semantics. Other prefixes ignore this choice.

Exact unsigned 32-bit arithmetic runs locally. This tool does not query, scan, locate, or test any address, and it does not configure a device.

Private address space

The three RFC 1918 blocks

These ranges are reserved for private internets. They are not globally routable public address space.

10.0.0.0/8

Large private block

10.0.0.0 through 10.255.255.255, containing 16,777,216 addresses.

172.16.0.0/12

Middle private block

172.16.0.0 through 172.31.255.255, containing 1,048,576 addresses.

192.168.0.0/16

Smaller private block

192.168.0.0 through 192.168.255.255, containing 65,536 addresses.

Private does not mean automatically secure. Firewalls, access controls, routing, and device configuration remain separate responsibilities.