CIDR Notation Explained: /24, /25, /26 and More

If you've seen something like 192.168.1.0/24 and wondered what on earth the "/24" means — you're in the right place. CIDR (pronounced "cider") notation is simply a shorthand way to write subnet masks.

The Simple Explanation

The number after the slash (/) tells you how many bits of the IP address are dedicated to the network portion. The rest identify individual devices within that network.

192.168.1.0/24
          ^^
          This /24 means: the first 24 bits (first 3 octets) are the NETWORK.

Since there are 32 bits total in an IPv4 address, that leaves 32 - 24 = 8 bits for devices. And 2⁸ = 256 addresses total (254 usable).

Common CIDR Prefixes and What They Mean

CIDR	Subnet Mask	Total IPs	Usable Hosts	When to Use
/8	255.0.0.0	16,777,216	16,777,214	Huge organizations
/16	255.255.0.0	65,536	65,534	Large networks
/24	255.255.255.0	256	254	Small office/home
/25	255.255.255.128	128	126	Medium-small networks
/26	255.255.255.192	64	62	Small teams
/27	255.255.255.224	32	30	Workgroups
/28	255.255.255.240	16	14	Small VLANs
/29	255.255.255.248	8	6	Point-to-point links
/30	255.255.255.252	4	2	Router connections
/32	255.255.255.255	1	0	Single host (used for routing)

Visualizing CIDR: The Pizza Analogy

Think of your network as a pizza:

/24 = Full pizza (256 slices) — serves 254 guests
/25 = Half pizza (128 slices) — serves 126 guests
/26 = Quarter pizza (64 slices) — serves 62 guests
/27 = Eighth pizza (32 slices) — serves 30 guests

Each step up (subtracting 1 from the CIDR number) cuts the pizza in half.

How to Read CIDR in Real Life

Home Network Example

Your router probably uses: 192.168.1.1/24
- This means: 192.168.1.0 through 192.168.1.255
- That's 254 devices you can connect
- Perfect for a typical home

Small Business Example

You need separate networks for:
- office: 10.0.1.0/24 (254 computers)
- IoT devices: 10.0.2.0/24 (254 sensors/devices)
- Guest wifi: 10.0.3.0/24 (254 guest devices)

Cloud Service Example (AWS VPC)

When creating an AWS VPC, you might choose:
- 10.0.0.0/16 (65,534 hosts) for the entire VPC
- Then break it into /24 subnets for different availability zones

Writing CIDR Notation Correctly

Here's the standard format:

[network address]/[CIDR prefix]

Examples:
192.168.1.0/24
10.0.0.0/16
172.16.0.0/12
8.8.8.0/24

The network address is typically written with zeros for the host portion (e.g., 192.168.1.0/24, not 192.168.1.100/24), though any address in the range works for calculation purposes.

IPv6: The Same Concept, Bigger Numbers

IPv6 uses 128 bits instead of 32, so you'll see CIDR like /64, /48, /32:

2001:db8::/32 - Documentation prefix
fe80::/10 - Link-local (like 169.254.0.0/16 in IPv4)
2001:db8:cafe::/48 - Your organization's prefix

The math stays the same — only the numbers get bigger!

Quick Reference Cheat Sheet

Want to support...	Use this CIDR
~16 million hosts	/8
~65,000 hosts	/16
~250 hosts	/24
~120 hosts	/25
~60 hosts	/26
~30 hosts	/27
~14 hosts	/28
2 routers only	/30

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