If you’ve ever seen an address written as 192.168.1.0/24 and wondered what the /24 means, that’s CIDR notation — Classless Inter-Domain Routing. It’s the standard way to describe how big a network is and which addresses belong to it.
The problem CIDR solved
Before CIDR (introduced in 1993), IP addresses were split into rigid “classes” — Class A, B, and C — each with a fixed, pre-defined size. A Class C network always had exactly 256 addresses, whether an organization needed 10 or 250 of them. This wasted enormous numbers of addresses and made routing tables bloated, since networks couldn’t be grouped efficiently. CIDR replaced that rigid system with a flexible one: any network can be any size, and that size is described directly in the notation.
Reading CIDR notation
A CIDR block has two parts: an IP address, and a slash followed by a number (the prefix length).
192.168.1.0/24
The prefix length tells you how many of the address’s 32 bits are fixed as the “network” portion — the rest are free for individual devices (“hosts”) on that network. A smaller number after the slash means a bigger network (more bits left for hosts); a larger number means a smaller network.
Why /24 is everywhere in home networking
A /24 network fixes the first 24 bits (the first three numbers: 192.168.1) and leaves the last 8 bits free — giving 256 possible addresses (0–255) in that last position. Subtract the network address itself and the broadcast address, and you get 254 usable addresses for actual devices. That’s more than enough for a home or small office, which is why almost every consumer router ships configured as a /24 network by default.
Common prefix lengths at a glance
| Prefix | Subnet mask | Total addresses | Usable hosts | Typical use |
|---|---|---|---|---|
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point links |
| /29 | 255.255.255.248 | 8 | 6 | Small server clusters |
| /28 | 255.255.255.240 | 16 | 14 | Small office segments |
| /24 | 255.255.255.0 | 256 | 254 | Home & small office networks |
| /16 | 255.255.0.0 | 65,536 | 65,534 | Large organizations |
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 | Entire private ranges (e.g. 10.0.0.0/8) |
Working out the details manually
To fully describe a /24 network like 192.168.1.0/24, there are four things worth knowing:
- Network address: 192.168.1.0 — identifies the network itself, not assignable to a device.
- Broadcast address: 192.168.1.255 — used to send a message to every device on the network at once, also not assignable.
- Usable range: 192.168.1.1 through 192.168.1.254 — the 254 addresses you can actually give to devices.
- Subnet mask: 255.255.255.0 — the older way of expressing the same /24 boundary, still used in some router interfaces.
Working this out by hand for anything other than a /24 gets tedious fast — our Subnet Calculator does it instantly for any prefix length, including the reverse: give it a subnet mask and it’ll tell you the CIDR prefix.
CIDR beyond home networks
CIDR isn’t just for subnetting — it’s also how internet routers describe which chunks of the internet they can reach, letting many individual networks be summarized as one routing table entry when they share a common prefix. That’s the “aggregation” part of the name, and it’s a big part of why the modern internet’s routing tables are manageable at all.
Frequently asked questions
What’s the difference between a subnet mask and a CIDR prefix?
They describe exactly the same boundary in two different formats — /24 and 255.255.255.0 mean the same thing. CIDR notation is more compact; subnet masks are the older, more verbose format still shown in some router UIs.
Can I use any prefix length I want on my home router?
Most consumer routers default to /24 and don’t expose subnetting controls at all. If yours does (usually under “LAN Setup” or “DHCP Settings”), you can typically choose a different one, but /24 is the right size for the vast majority of home networks.
Why do some addresses in a range show as unusable?
Every subnet reserves its first address (the network address) and its last address (the broadcast address) for network functions — they’re never assigned to a device, which is why a /24 gives you 256 total addresses but only 254 usable ones.