Learn how Classless Inter-Domain Routing (CIDR) works and how it simplifies IP addressing and subnetting.
CIDR stands for Classless Inter-Domain Routing. It is the modern method of defining IP address ranges using a prefix length instead of the older Class A, B and C network system. Instead of writing a subnet mask like 255.255.255.0, CIDR lets us write the much shorter /24. Today, CIDR is used by virtually every modern network including enterprise networks, cloud providers, Internet Service Providers (ISPs) and home routers.
A CIDR prefix tells you how many bits belong to the network portion of an IP address. For example:
| CIDR | Subnet Mask | Usable Hosts |
|---|---|---|
| /30 | 255.255.255.252 | 2 |
| /29 | 255.255.255.248 | 6 |
| /28 | 255.255.255.240 | 14 |
| /27 | 255.255.255.224 | 30 |
| /26 | 255.255.255.192 | 62 |
| /25 | 255.255.255.128 | 126 |
| /24 | 255.255.255.0 | 254 |
| /23 | 255.255.254.0 | 510 |
| /22 | 255.255.252.0 | 1022 |
| /16 | 255.255.0.0 | 65,534 |
CIDR makes networks much more efficient by allowing administrators to create subnets of virtually any size. Instead of wasting thousands of IP addresses under the old classful networking model, CIDR allows organizations to allocate only the addresses they actually need. This flexibility is one of the reasons the Internet has been able to continue growing despite the limited IPv4 address space.
Need to calculate a subnet? Use our free calculator to instantly determine:
Continue learning networking fundamentals: