What Is a CIDR Block? IP Subnetting Explained Simply

Why You Need to Understand CIDR

CIDR (Classless Inter-Domain Routing) notation appears everywhere in modern networking:

• AWS VPC and security group rules
• Docker and Kubernetes network configs
• Firewall allow/deny rules
• Cloud provider IP range allowlisting
• Router configuration

If you have ever seen 10.0.0.0/16 or 192.168.1.0/24 and wondered what the /16 or /24 means, this guide will make it click.

IP Address Basics

An IPv4 address is 32 bits long, written as four groups of 8 bits (octets) in decimal, separated by dots:

192.168.1.100

In binary, each octet is 8 bits, so the full address is:

11000000.10101000.00000001.01100100

There are 2³² = ~4.3 billion possible IPv4 addresses.

What the Slash Means

The /n in CIDR notation specifies how many leading bits of the address are the network prefix, the fixed part that identifies the network. The remaining bits identify individual hosts within that network.

192.168.1.0/24

• /24 means the first 24 bits are the network prefix
• The remaining 8 bits (32 − 24) identify hosts
• Number of addresses: 2⁸ = 256 (ranging from 192.168.1.0 to 192.168.1.255)

CIDR Reference Table

CIDR	Prefix bits	Host bits	Addresses	Usable hosts
/32	32	0	1	1 (single host)
/31	31	1	2	2 (point-to-point links)
/30	30	2	4	2
/29	29	3	8	6
/28	28	4	16	14
/27	27	5	32	30
/26	26	6	64	62
/25	25	7	128	126
/24	24	8	256	254
/23	23	9	512	510
/22	22	10	1,024	1,022
/16	16	16	65,536	65,534
/8	8	24	16,777,216	16,777,214
/0	0	32	All addresses	All

"Usable hosts" is 2 fewer than total addresses: the network address (all host bits 0) and the broadcast address (all host bits 1) are reserved.

The Subnet Mask

The subnet mask is the network prefix expressed differently, as a 32-bit number with the network bits set to 1 and host bits set to 0.

/24 → subnet mask 255.255.255.0 (24 ones followed by 8 zeros) /16 → subnet mask 255.255.0.0 /8 → subnet mask 255.0.0.0

Both notations express the same thing. CIDR (/24) is more compact and modern; subnet masks (255.255.255.0) appear in older documentation and some device UIs.

Common Real-World CIDR Blocks

Private IP ranges (RFC 1918):

• 10.0.0.0/8, large private network (16M addresses)
• 172.16.0.0/12, medium private network (1M addresses)
• 192.168.0.0/16, home network default (65K addresses)

Common cloud configurations:

• A VPC 10.0.0.0/16 contains 65,536 addresses, split into subnets like 10.0.1.0/24 (256 each)
• A security group rule 0.0.0.0/0 means "all IPv4 addresses" (allow from anywhere)
• x.x.x.x/32 means exactly one specific IP address

Calculating Subnets

To find the network address and broadcast address for a CIDR block:

1. Convert the IP to binary
2. Apply the prefix length: the first n bits are fixed (network), the rest are host bits
3. Network address: all host bits = 0
4. Broadcast address: all host bits = 1
5. First usable host: network address + 1
6. Last usable host: broadcast address − 1

Example: 192.168.5.0/28

• /28 → 28 network bits, 4 host bits
• Addresses: 2⁴ = 16 (192.168.5.0 to 192.168.5.15)
• Network: 192.168.5.0
• Broadcast: 192.168.5.15
• Usable: 192.168.5.1 to 192.168.5.14 (14 hosts)

The CIDR / Subnet Calculator on this site calculates all these values instantly from any CIDR input.

Summary

CIDR notation IP/n splits an IP address into a fixed network prefix (n bits) and variable host bits (32-n bits). The number of addresses in the block is 2^(32-n). /24 = 256 addresses, /16 = 65,536, /8 = 16M. Understanding this makes cloud network configuration, firewall rules, and routing decisions readable and logical.