What Is CIDR? Classless IP Address Notation Explained

What Is CIDR?

CIDR (Classless Inter-Domain Routing) is the modern way to describe IP address ranges. Instead of legacy "Class A/B/C" networks, CIDR uses a prefix length — the number of leading bits that identify the network.

Written as network/prefix, for example:

192.168.1.0/24
10.0.0.0/8
172.16.0.0/12

The number after the slash tells you how many bits belong to the network portion. Everything after that is available for hosts.

CIDR vs subnet mask

CIDR and subnet masks express the same boundary in different formats:

CIDR	Subnet mask	Host bits	Usable hosts
/8	255.0.0.0	24	16 777 214
/16	255.255.0.0	16	65 534
/24	255.255.255.0	8	254
/26	255.255.255.192	6	62
/30	255.255.255.252	2	2 (point-to-point)
/32	255.255.255.255	0	1 (single host)

A /24 means the first 24 bits are fixed (the network) and the last 8 bits vary (hosts). That gives 2⁸ = 256 addresses, minus network and broadcast = 254 usable hosts.

How prefix length works

Each IPv4 address is 32 bits. The prefix length counts network bits from the left:

192.168.1.100/24

192  = 11000000  ← network bits (24 total)
168  = 10101000  ← network bits
1    = 00000001  ← network bits
100  = 01100100  ← host bits (last 8)

The network address zeroes out all host bits: 192.168.1.0. The broadcast address sets all host bits to 1: 192.168.1.255.

Use the Subnet / CIDR Calculator to compute these instantly from any IP and prefix.

Why CIDR replaced classful addressing

Before CIDR (pre-1993), IP addresses were divided into fixed classes:

Class	First octet	Default mask	Problem
A	0–127	/8	Wasted millions of addresses per network
B	128–191	/16	Too large for most orgs
C	192–223	/24	Too small for many orgs

CIDR allows any prefix length, so you can allocate exactly the block size you need — a /27 for 30 hosts, a /22 for 1 022 hosts, and so on.

Common CIDR blocks in practice

Private RFC 1918 ranges (not routable on the public internet):

• 10.0.0.0/8 — one large block (16 M addresses)
• 172.16.0.0/12 — 16 contiguous /16 blocks
• 192.168.0.0/16 — the familiar home/office range

Cloud VPC defaults:

• AWS default VPC: often 172.31.0.0/16
• Azure VNet: commonly 10.0.0.0/16 or /24 subnets inside
• GCP VPC: 10.128.0.0/9 auto mode range

Loopback and special:

• 127.0.0.0/8 — localhost
• 169.254.0.0/16 — link-local (APIPA)

Subnetting with CIDR

To split a larger block into smaller subnets, increase the prefix length:

Parent:  192.168.0.0/16  (65 534 hosts)
Split:   192.168.0.0/24  (254 hosts each)
         192.168.1.0/24
         …
         192.168.255.0/24

Each step of +1 on the prefix halves the address space. Going from /16 to /24 adds 8 bits → 2⁸ = 256 subnets.

Use the CIDR Subnet Splitter to generate the full table of child subnets.

Wildcard masks

In Cisco ACLs and some firewall rules, the wildcard mask is the inverse of the subnet mask:

Subnet mask	Wildcard mask
255.255.255.0	0.0.0.255
255.255.255.192	0.0.0.63
255.255.0.0	0.0.255.255

Wildcard 0 means "must match"; 1 means "don't care."

Quick reference: hosts per prefix

Prefix	Total addresses	Usable hosts
/30	4	2
/29	8	6
/28	16	14
/27	32	30
/26	64	62
/25	128	126
/24	256	254
/23	512	510
/22	1 024	1 022

Related tools

• Subnet / CIDR Calculator — network, broadcast, and host range for any IP + prefix
• CIDR Subnet Splitter — divide a parent block into equal subnets
• IP Address Converter — convert between decimal, binary, hex, and integer