What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation is a way of representing IP address ranges using a slash prefix, such as /24 or /27.

It defines how many bits of an IP address belong to the network portion, with the remaining bits used for host addresses.

The number after the slash indicates how many bits are reserved for the network portion of the address.

For example, a /24 means 24 bits are fixed for the network, leaving 8 bits available for hosts.

192.168.1.0/24

Network bits: 24
Host bits: 8
Total hosts: 256 (254 usable)

• /24 → 255.255.255.0
• /25 → 255.255.255.128
• /27 → 255.255.255.224
• /30 → 255.255.255.252

A CIDR block represents a continuous range of IP addresses.

192.168.1.0/27

Range: 192.168.1.0 - 192.168.1.31
Usable: 192.168.1.1 - 192.168.1.30

CIDR notation is widely used in firewall rules, routing tables, cloud networking, and subnet design.

It provides a compact and consistent way to describe IP ranges without needing full subnet mask notation.

• Firewall allow/deny rules
• Azure and AWS virtual networks
• VPN configurations
• Routing tables
• Access control lists (ACLs)

You can calculate CIDR ranges and subnet boundaries using DNS Pro: https://app.dnspro.co.uk

# Example using ipcalc
ipcalc 192.168.1.0/27

If you're new to networking fundamentals, start here: /articles/what-is-dns