CIDR notation (Classless Inter-Domain Routing) is the standard shorthand for describing a whole block of IP addresses at once, rather than listing every individual address.
How to read it
A CIDR block looks like `192.168.1.0/24`. The part after the slash — the prefix length — tells you how many of the address's leading bits are fixed (the "network" portion), with the rest available for individual host addresses within that block.
- `/24` fixes the first 24 bits, leaving 8 bits free → 256 total addresses (a typical small home/office network range).
- `/16` fixes 16 bits, leaving 16 free → 65,536 addresses.
- `/8` fixes just 8 bits → 16.7 million addresses.
A smaller number after the slash means a larger address range — this trips a lot of people up at first.
Why it matters
CIDR is how routers decide where to send traffic, how firewall rules are scoped ("allow this whole office network," not one IP at a time), and how registries describe the blocks of addresses allocated to an ISP or organization. Understanding it is genuinely useful for reading firewall configs, cloud security groups, or an IP WHOIS record's allocation range.
Working it out by hand vs. using a calculator
The math (subnet mask, broadcast address, usable host range) is mechanical but easy to get wrong under pressure — which is exactly what a calculator is for.
How to check it yourself
Our CIDR Calculator takes any CIDR block and instantly shows the network address, broadcast address, subnet mask, and full usable address range. For the CIDR block a specific IP belongs to, see its network details on our IP Lookup tool.