What is CIDR Notation?

CIDR stands for Classless Inter-Domain Routing. It is the modern standard used to allocate IP addresses and route internet traffic efficiently. Instead of using clunky "Classes" (like Class A, B, or C networks), CIDR uses a slash followed by a number (like /24) to define exactly how large a network is.

An IPv4 address is made up of 32 binary bits. The CIDR number simply tells the router how many of those 32 bits belong to the network. The remaining bits are left over for you to assign to individual devices (hosts) like computers, servers, and phones.

How Subnetting Limits Usable Hosts

When you carve out a network, you can't actually use every single IP address in that range. Two addresses are always reserved by the networking protocols:

• The Network Address (The First IP): Used to identify the network itself. It cannot be assigned to a device.
• The Broadcast Address (The Last IP): Used to send data to every single device on that network simultaneously.

This is why a /24 network, which mathematically contains 256 IP addresses, only provides 254 usable hosts for your devices.

Common Cloud & Local Subnets

If you are building virtual private clouds (VPCs) in AWS, Google Cloud, or managing a home router, you will encounter these standard CIDR blocks: