Get existing or find the next available Subnet on an Azure VNET

getOrFindNextSubnet.ps1

param (
    $subnetName,
    $vnetGroupName,
    $vnetName,
    $subnetMask
)

Write-Host "subnetName '$subnetName'" -ForegroundColor Green 
Write-Host "vnetGroupName '$vnetGroupName'" -ForegroundColor Green 
Write-Host "vnetName '$vnetName'" -ForegroundColor Green 
Write-Host "subnetMask '$subnetMask'" -ForegroundColor Green 

$names = az network vnet subnet list -g $vnetGroupName --vnet-name $vnetName --query [].[name,addressPrefix] -o json
$nameObject = [system.String]::Join(" ", $names) | ConvertFrom-Json

foreach ($name in $nameObject)
{
    $azureName = $name[0]
    $address = $name[1]

    if ($azureName -eq $subnetName) {
        Write-Host "Found existing subnet: $address. Setting env variable 'Devops:Terraform:CIDR'" -ForegroundColor Green 
        echo "##vso[task.setvariable variable=Devops:Terraform:CIDR]$address"
        exit 0
    }
}

for ($io = 0; $io -lt 255; $io+=1)
{
    # grab the first address on increments on
    for ($i = 0; $i -lt 255; $i+=32)
    {
        $found = $false
        $cidr =  [string]::Format($subnetMask, $io, $i) #10.100.{0}.{1}/27

        foreach ($name in $nameObject)
        {
            if ($cidr -eq $name[1]) {
                $found = $true
            }
        }

        if ($found -eq $false) {
            Write-Host "Claiming available subnet: $cidr. Setting env variable 'Devops:Terraform:CIDR'" -ForegroundColor Green 
            echo "##vso[task.setvariable variable=Devops:Terraform:CIDR]$cidr"
            exit 0
        }
    }
}

Write-Host "No subnets available to be created in $subnetMask" -ForegroundColor Green 
exit 1

I run this via a Azure CLI Task in Azure DevOps. When it finds or identifies the next available subnet, it sets an environment variable for Terraform to pickup called ‘DevOps:Terraform:CIDR’.

For a passed in subnet mask like: 10.100.{0}.{1}/27

This will loop the third octet by one and the fourth octet by 32 given it is a /27 CIDR passed in.

You could probably calculate this out based on the CIDR passed in but we knew what size we needed, so was fine to hardcode it given PaaS services in Azure can only join a /27 as minimum.

Hope it helps someone.