CIDR Notation Explained: What /24 Actually Means

If you’ve configured a firewall rule, set up a proxy pool, or even just looked at your router’s network settings, you’ve probably seen an IP range written like 192.168.1.0/24. That slash-number is CIDR notation, and once it clicks, it makes reading any network configuration significantly easier.

What the number after the slash means

The number is how many bits, out of 32 total in an IPv4 address, are fixed as the “network” portion - everything after that is available for individual host addresses. A /24 fixes the first 24 bits (the first three numbers: 192.168.1), leaving 8 bits free - 256 possible values (0-255) for the last number. A smaller number after the slash means a bigger range: a /16 only fixes 16 bits, leaving 65,536 addresses available instead of 256.

The two addresses you can’t actually use

Every subnet reserves exactly two addresses that can’t be assigned to a device: the network address (all host bits set to 0 - it identifies the subnet itself) and the broadcast address (all host bits set to 1 - it’s used to send a message to every device on the subnet at once). For a /24, that means a 256-address block only has 254 usable addresses for actual devices.

Why this matters practically

Whenever you’re setting up a proxy server, writing firewall rules, or dividing a network into smaller segments, you need to know exactly which addresses belong to a given block, and which two you can’t assign. Getting this wrong is a common source of “why won’t this device get an IP” bugs - usually because someone tried to assign the network or broadcast address by mistake.

Doing the math without doing the math

Our Subnet / CIDR Calculator takes any IPv4 CIDR block and instantly returns the network address, broadcast address, usable host range, and total host count - the arithmetic above, done for you. If you need to check your own current IP before working out which subnet it falls in, our What’s My IP tool is the fastest way to get it.