What does CIDR stand for?

CIDR stands for Classless Inter-Domain Routing (pronounced 'cider'). It was introduced in 1993 by RFC 1519 to replace the old classful networking system (Class A, B, C). CIDR allows network prefixes of any length from /0 to /32, rather than being locked to /8, /16, or /24.

What does /24 mean in an IP address?

The /24 in an IP address like 192.168.1.0/24 means that the first 24 bits of the 32-bit IP address identify the network, and the remaining 8 bits identify individual hosts. This gives 2^8 = 256 total addresses, with 254 usable for hosts. /24 is equivalent to the subnet mask 255.255.255.0.

How do I convert a CIDR prefix to a subnet mask?

To convert a CIDR prefix to a subnet mask, write 32 binary bits with the first N bits set to 1 and the remaining bits set to 0, then convert each 8-bit group to decimal. For /24: 11111111.11111111.11111111.00000000 = 255.255.255.0. For /20: 11111111.11111111.11110000.00000000 = 255.255.240.0.

CIDR Notation Explained

Last reviewed: May 2026

CIDR notation (pronounced "cider") is the standard way to write an IP address with its network size. You've seen it everywhere: 192.168.1.0/24, 10.0.0.0/8, 172.16.0.0/12. This guide explains exactly what the slash number means, how it relates to subnet masks, and why the entire internet depends on it.

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What Does the Slash Number Mean?

An IPv4 address is 32 bits long. The slash number — the prefix length — tells you how many of those 32 bits identify the network. The remaining bits identify individual hosts within that network.

11111111.11111111.11111111.00000000 ← /24 (24 network bits, 8 host bits)
11111111.11111111.00000000.00000000 ← /16 (16 network bits, 16 host bits)
11111111.00000000.00000000.00000000 ← /8 (8 network bits, 24 host bits)

Network bits (fixed — same for all hosts in the subnet) Host bits (variable — each host gets a unique value)

The number of host addresses is 2^{(32 − prefix)}. For a /24: 2^(32−24) = 2⁸ = 256 addresses. Two are always reserved (network + broadcast), leaving 254 usable.

Interactive CIDR Prefix Explorer

Prefix length: /24

CIDR vs Subnet Mask — Two Ways to Say the Same Thing

CIDR prefix length and dotted-decimal subnet mask are equivalent representations. The subnet mask is formed by setting the first N bits to 1 and the rest to 0, then converting each 8-bit group to a decimal number.

CIDR Prefix	Subnet Mask	Total IPs	Usable Hosts
/8	255.0.0.0	16,777,216	16,777,214
/16	255.255.0.0	65,536	65,534
/20	255.255.240.0	4,096	4,094
/24	255.255.255.0	256	254
/26	255.255.255.192	64	62
/28	255.255.255.240	16	14
/30	255.255.255.252	4	2
/32	255.255.255.255	1	1 (host route)

How CIDR Replaced Classful Networking

Before 1993, IPv4 used classful networking: addresses were divided into Class A (/8), Class B (/16), and Class C (/24) based on the first octet. This was wasteful — a company needing 500 hosts had to get an entire Class B (65,534 hosts), wasting 65,000+ addresses.

Class	First Octet Range	Prefix	Addresses	Problem
A	1–126	/8	16,777,214	Massively over-sized for most orgs
B	128–191	/16	65,534	Over-sized; wasted millions of IPs
C	192–223	/24	254	Too small for many organisations

CIDR solved the "Class B Problem"

With CIDR (RFC 1519, 1993), any prefix length from /0 to /32 is valid. A company needing 500 hosts gets a /23 (510 usable hosts) instead of a /16. This drastically slowed IPv4 exhaustion and enabled the modern internet's routing table to be manageable.

Reading a CIDR Block: Step by Step

Take 192.168.10.0/24 as an example:

192.168.10.0 — the network address (first address in the block)
/24 — 24 bits are the network; 8 bits are for hosts
Host range — .1 through .254 (usable hosts)
Broadcast — 192.168.10.255 (last address; not assignable)
Subnet mask — 255.255.255.0 (24 ones followed by 8 zeros)

CIDR Aggregation (Supernetting)

CIDR also allows you to aggregate multiple contiguous subnets into a single, shorter-prefix route — called a supernet or summary route. This is essential for keeping BGP routing tables small.

192.168.0.0/24 → 11000000.10101000.00000000.00000000 192.168.1.0/24 → 11000000.10101000.00000001.00000000
Both share 23 common prefix bits → summarise as 192.168.0.0/23

RFC 1918 — The Private CIDR Blocks

Three CIDR ranges are reserved for private use and will never appear on the public internet (defined in RFC 1918):

Block	Range	Total Addresses	Common Use
10.0.0.0/8	10.0.0.0 – 10.255.255.255	16,777,216	Large enterprises, cloud VPCs
172.16.0.0/12	172.16.0.0 – 172.31.255.255	1,048,576	Corporate networks, Docker defaults
192.168.0.0/16	192.168.0.0 – 192.168.255.255	65,536	Home routers, small office networks

CIDR in Cloud Networking

AWS VPC — VPC CIDR must be /16 to /28. Each subnet within the VPC has its own CIDR, and AWS reserves 5 IPs per subnet.
Azure VNet — VNet CIDRs are flexible; subnets are carved out of the VNet's address space. Azure also reserves 5 IPs per subnet.
GCP VPC — Global VPCs with regional subnets. GCP reserves 4 IPs per subnet. Secondary CIDR ranges support GKE pod and service IPs.
Kubernetes — Pod and Service CIDRs are separate from node CIDRs. A /16 for pods is typical (65,534 IPs for containers).

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