CIDR

CIDR (Classless Inter-Domain Routing) expresses a network as IP/prefix length (e.g., 10.0.0.0/8), replacing IPv4 address classes with flexible network partitioning.

Purpose

It provides more flexible network partitioning and route aggregation than address classes, reducing routing table bloat.

Mechanism

CIDR notation is IP/n, where n is the number of network prefix bits (0-32 for IPv4, 0-128 for IPv6); for example, /24 means the first 24 bits form the network number. Route aggregation (supernetting) merges multiple /24s into a /23 to shrink routing table entries.

Use cases

Routing tables and route aggregation
Firewall rules
Cloud security groups
IP allowlists

Examples

Input
```
10.0.0.0/8
```
Output
```
10.0.0.0 - 10.255.255.255
```
16.77 million addresses

Common errors

Prefix length exceeds the address bit count: Keep it within 32 for IPv4 and 128 for IPv6

Related terms

A subnet is a technique for dividing a large network into smaller ones, using the subnet mask to separate the network number from the host number, improving address utilization and isolation.

IPv4 is the 32-bit network addressing protocol, written in dotted decimal (e.g., 192.168.1.1), with roughly 4.3 billion addresses—now exhausted.

IPv6 is the 128-bit network addressing protocol, written in colon-separated hexadecimal (e.g., 2001:db8::1), offering about 3.4×10^38 addresses to solve IPv4 exhaustion.

Related tools

IP Address Info

Analyze IPv4/IPv6 addresses and CIDR subnets: class, private/multicast flags, binary/hex, reverse DNS, network/broadcast/hosts. Free, private, and 100% client-side.

Cidr To Range — pure client-side tool. Privacy-first, no server round-trip.

Range To Cidr (Aligned Only)

Range To Cidr — pure client-side tool. Privacy-first, no server round-trip. Simplified tool: succeeds only when the range exactly matches one aligned CIDR block (e.g. 10.0.0.0-10.0.0.255 → /24); non-aligned ranges are rejected — this is not a general range-to-CIDR splitter.

Netmask Calculator

Netmask Calculator — pure client-side tool. Privacy-first, no server round-trip.

Wildcard Mask Calculator

Wildcard Mask Calculator — pure client-side tool. Privacy-first, no server round-trip.

References