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Subnet calculator

Enter an address and CIDR prefix, get every subnet fact instantly in your browser.

How Subnet calculator works

Subnet calculator takes an IPv4 address with a CIDR prefix (like 192.168.1.10/24) and works out every derived fact locally in your browser: the network and broadcast address, the subnet mask and its wildcard form, the first and last usable host, the total and usable address counts, and whether the network falls in a private (RFC 1918) range. Nothing you type is sent anywhere.

The arithmetic is exact 32-bit bitwise math, not an approximation: masks are built from the prefix length, the network address masks off the host bits, and the broadcast address sets them all to one. Point-to-point /31 links (RFC 3021, both addresses usable) and single-host /32 entries are handled as their own special cases rather than forced through the usual "subtract network and broadcast" formula.

Written by Bastien Sulyan

How to use Subnet calculator, step by step

  1. Type an IPv4 address followed by a slash and a CIDR prefix, for example 10.0.0.5/16.
  2. Read the network address, broadcast address, and subnet/wildcard masks from the result grid.
  3. Check the first and last usable host addresses for that network.
  4. Note the total and usable address counts, and whether the network is a private (RFC 1918) range.

Common use cases

  • Working out the usable host range for a new /24 before assigning static IPs on a home or office LAN.
  • Checking whether an address a colleague pasted in chat is inside a private range before treating it as internal.
  • Confirming the broadcast address of a subnet before configuring a firewall rule.
  • Studying for a networking certification and verifying CIDR math by hand against the calculator.

Frequently asked questions

Does this calculator send the address I type to a server?

No. Every computation is plain 32-bit bitwise arithmetic that runs in your browser tab as you type. The address and prefix never leave your device, and the page keeps working if you disconnect from the internet after it loads.

What happens if I omit the CIDR prefix?

The prefix defaults to /32, treating the input as a single host: the network, broadcast, first and last host all equal that one address, and the total/usable address count is 1.

How are /31 and /32 networks handled?

A /32 is a single host: network, broadcast and the only usable address are identical. A /31 (RFC 3021, common on point-to-point router links) has no separate network/broadcast pair usable as hosts, so BOTH of its two addresses are reported as usable, unlike every other prefix length.

How does the private-range (RFC 1918) check work?

The calculator checks whether the network address falls inside the three ranges IANA reserves for private use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. It only reports whether the address matches one of those three ranges, nothing about routing or actual reachability.

Does this support IPv6?

Not yet. This version handles IPv4 CIDR notation only (0 to 32-bit prefixes). An IPv6 address or prefix will be rejected as an invalid octet.

What counts as an invalid address?

Any input that is not four dot-separated numbers from 0 to 255, or a prefix that is not a whole number from 0 to 32, is rejected with a specific error message rather than silently guessed at.