Azure Subnet Calculator

Introduction

AzureSubnetCalculator.ps1 is a powerful and easy-to-use PowerShell tool designed to simplify subnet planning in Microsoft Azure environments.
Azure networking has specific rules regarding reserved IP addresses and usable host calculations, which differ from traditional on-premises networks.
This script helps students, engineers, architects, and administrators quickly calculate subnets that fully respect Azure requirements.

Whether you are designing a new virtual network, planning subnet expansions, or validating an existing cloud architecture — this tool provides accurate, fast, and Azure-compliant subnet information.

Requeriments

You can download the script from my GitHub repository: https://github.com/az104tor/AzureSubnetCalculator

To run it, the following requirements apply:

– Supported Operating Systems
  • Windows 10 or Windows 11
  • Windows Server 2016 or later
  • Linux distributions with PowerShell 7 installed
  • macOS with PowerShell 7 installed
– PowerShell Version

Recommended:

  • PowerShell 7.0+ (pwsh)

Full ANSI color support and best performance, compatibility.

Minimum:

  • Windows PowerShell 5.1

Script runs correctly, but color output may require additional configuration

To enable ANSI colors (optional):

Set-ItemProperty HKCU:\Console VirtualTerminalLevel -Type DWORD 1

– Execution Policy

Because the script is not digitally signed, Windows may block it.
Users must configure PowerShell to allow local scripts:

Set-ExecutionPolicy RemoteSigned  # allow always local unsigned scripts

Or allow only this powershell session:

Set-ExecutionPolicy -Scope Process -ExecutionPolicy Bypass   #allow temporale the execution of unsigned scripts

Also, when a script is downloaded from the internet, user must umblock the file. Move to the Download directory and run:

Unblock-File -Path .\AzureSubnetCalculator.ps1
– Privileges

No administrative rights are required to run the script. Administrative rights are required only if the user wants to change the execution policy for the system (see above).

– Input Requirements

The script requires a valid IPv4 CIDR (Classless Inter-Domain Routing) notation:

Examples:

10.0.0.0/16

192.168.1.0/24

172.16.10.0/27

Invalid inputs (missing CIDR or malformed IPs) will generate an error. Please refer to RFC 1918 for Best Current Practice.

Script functionalities

1) Calculates all essential subnet details:
  • Network address
  • Broadcast address
  • Subnet mask
  • CIDR prefix
  • Total hosts (raw IPv4 capacity)
2) Azure-Specific Enhancements:
  • Identifies Azure reserved IPs:
    • .1 → default gateway
    • .2 and .3 → reserved
    • .255 (if applicable) → broadcast
  • Calculates Azure-usable hosts (total minus 5)
  • Displays the usable host range (accurate for Azure constraints)
  • Display a warning if the CIDR prefix is not supported in Azure
3) CIDR Recommendation Engine (Optional):

Automatically recommends the smallest subnet that fits a required number of hosts.

-HostsNeeded 120

→ Recommended CIDR: /25
4) CIDR Recommendation Engine (Optional):

Easily split any subnet into multiple smaller subnets.
Example:

-CIDR 10.0.0.0/24 -SplitSubnets 4

→ Produces four /26 subnets

Script Usage

Basic subnet calculation:
./AzureSubnetCalculator.ps1 -CIDR 192.168.0.0/25
Azure-aware CIDR recommendation:
./AzureSubnetCalculator.ps1 -HostsNeeded 300
Split a subnet into four:
./AzureSubnetCalculator.ps1 -CIDR 10.5.0.0/22 -SplitSubnets 4
Examples of invalid and Azure no compliant CIDR:

License & Contribution

You are free to use, modify, and distribute this script. Pull requests, suggestions, and improvements are welcome!