CIDR Notation Explained: How IP Addressing Works

CIDR Notation Explained

CIDR (Classless Inter-Domain Routing) notation is the standard way to express IP addresses and their associated network masks. Introduced in 1993 (RFC 1519), it replaced the rigid classful addressing system (Class A, B, C) with flexible, variable-length subnetting.

The Format

CIDR notation consists of an IP address followed by a slash and a number:

10.0.0.0/16

• 10.0.0.0 — the network (or host) address
• /16 — the prefix length, indicating how many leading bits define the network

What the Prefix Length Means

The number after the slash tells you how many of the 32 bits (in IPv4) are the network portion. The remaining bits are the host portion.

Prefix	Subnet Mask	Hosts
/8	255.0.0.0	16,777,214
/16	255.255.0.0	65,534
/24	255.255.255.0	254
/32	255.255.255.255	1

Why CIDR Replaced Classful Addressing

Classful addressing only allowed three fixed sizes: /8 (Class A), /16 (Class B), and /24 (Class C). This was extremely wasteful. An organization needing 500 addresses would receive a Class B (/16) with 65,534 addresses — wasting over 65,000 addresses.

CIDR allows any prefix length from /0 to /32, so networks can be sized precisely to their needs. A /23 gives 510 hosts, a /22 gives 1,022, and so on.

CIDR in Practice

CIDR is used everywhere in modern networking: routing tables, firewall rules, cloud VPC configurations, DNS ACLs, and more. Understanding CIDR is fundamental to working with any IP network, from a home lab to a global enterprise architecture.