Read a /24 address and know your network's size

192.168.1.0/24 on a config page, a router asking for a subnet mask, a firewall rule mentioning /32. What CIDR notation means and how to get the full picture.

Tools used in this guide

IPv4 subnet calculator

The job

You found a CIDR block — 10.0.0.0/8 in a cloud config, 192.168.1.0/24 on a home router — and need to know what it covers: the range it includes, how many addresses, which are usable.

What the number means

The /n counts the leading bits that stay fixed. /24 fixes the first three octets — 192.168.1.0/24 covers 192.168.1.0 through 192.168.1.255. Each step down doubles the network: /25 is half a /24, /26 a quarter. Enter the block into the calculator to see the mask, the range, and the usable count.

/24 = 256 addresses, 254 usable (network and broadcast are reserved).
/30 = 4 addresses, 2 usable — the classic point-to-point link.
/31 = the RFC 3021 exception: 2 usable, no broadcast.
/32 = a single host, used in firewall rules for one machine.

Worked example

Enter 10.20.30.40/26 and the calculator returns network 10.20.30.0, broadcast 10.20.30.63, mask 255.255.255.192, 62 usable addresses (10.20.30.1 – .62). An address entered without a prefix defaults to /24 — add the slash when you know it.

Limits

IPv4 only — IPv6 uses the same slash notation but a different calculator. Subnet masks must be contiguous: 255.255.0.255 is not a real mask and is rejected. The address you give is normalized to its network base (10.20.30.40/26 → 10.20.30.0/26).