How to Calculate Subnet Masks and IP Ranges from CIDR Notation

In cloud architecture (AWS VPCs, Azure Virtual Networks, Google Cloud) and enterprise networking, CIDR notation (Classless Inter-Domain Routing) is the universal standard for allocating IP address blocks.

Understanding how a CIDR prefix (such as /24 or /16) translates into usable host IP addresses, subnet masks, and network boundaries is a core competency for systems engineers and web developers.

This guide breaks down IPv4 binary math, explains how to determine usable IP addresses, and links to the CalculatorAll Subnet & CIDR Calculator.

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What Does the Slash (/) in CIDR Represent?

An IPv4 address consists of 32 binary bits divided into four 8-bit octets (e.g., 192.168.1.0).

The slash notation (/prefix) specifies how many leading bits are allocated to the Network ID, leaving the remaining bits for Host addresses:

Host Bits (h) = 32 - CIDR Prefix

Total IP Addresses = 2^h = 2^(32 - Prefix)

Usable Host IP Addresses = 2^h - 2

(Note: We subtract 2 because the very first IP is the Network Identifier and the very last IP is the Broadcast Address).

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Step-by-Step Calculation: 192.168.1.0/24

Suppose you are configuring a subnet with the CIDR block 192.168.1.0/24:

1. Calculate Host Bits: 32 - 24 = 8 bits for hosts.
2. Calculate Total IPs: 2⁸ = 256 total IP addresses.
3. Calculate Usable IPs: 256 - 2 = 254 usable host addresses.
4. Determine the Subnet Mask: The first 24 bits are set to 1 in binary (11111111.11111111.11111111.00000000), which in decimal is 255.255.255.0.
5. Determine the Address Range:
   • Network ID: 192.168.1.0
   • First Usable Host: 192.168.1.1
   • Last Usable Host: 192.168.1.254
   • Broadcast Address: 192.168.1.255

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CIDR IPv4 Subnet Cheat Sheet