Can I find the amount of available IP addresses in an Azure subnet?

Question

I'm trying to find a way to list the amount of available IP addresses in an Azure vNet. I know this is available in the GUI, as can be seen below:

Question is, can we have the same information in the Powershell Az module?

Thanks!

Eitan

Answer 1

From what I've seen, there is no direct way to fetch the number of available IP addresses from a subnet address range.

However, its easy enough to use the following formula:

2 ^ (32 - netmask length) - number of reserved Azure IPs - number of already occupied IPs

So if you had a subnet address range such as 10.1.0.0/24, and one IP was already being occupied by another device, this would calculate to:

(2 ^ (32 - 24)) - 5 - 1 => (2 ^ 8) - 6 => 256 - 6 => 250

Furthermore, with PowerShell we could put these steps into code like this:

$resourceGroupName = "myResourceGroup"
$virtualNetworkName = "myResourceGroup-vnet"
$subnetName = "default"

$virtualNetwork = Get-AzVirtualNetwork `
    -Name $virtualNetworkName `
    -ResourceGroupName $resourceGroupName

$subnet = Get-AzVirtualNetworkSubnetConfig `
    -VirtualNetwork $virtualNetwork `
    -Name $subnetName

$subnetMask = $subnet.AddressPrefix.Split("/")[1]

$netmaskLength = [Math]::Pow(2, 32 - [int]$subnetMask)

$availableIpAddresses = $netmaskLength - 5 - $subnet.IpConfigurations.Count

$availableIpAddresses

Reference for the above Az module Cmdlets:

• Get-AzVirtualNetwork
• Get-AzVirtualNetworkSubnetConfig

Additionally, have look at MSDN to see why Azure reserves 5 IP addresses specifically.

Answer 2

I came across this solution,

$TableProperties = @(
    @{Name="VNET" ; Expression={$VNET.Name}}
    "Name"
    "AddressPrefix"
    @{Name="IPsConfigured" ; Expression={$SubnetConfigured.Count}}
    @{Name="IPsLeft" ; Expression={$AvailableAddresses - $SubnetConfigured.Count}} 
)

$Subnet | 
    Select-Object $TableProperties | 
        Format-Table -Autosize