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How to automate IP ranges calculations in Azure using PowerShell

2023-10-01

Suppose we have been allocated the IP range of 10.172.0.0/16 by the network team for planned Azure Landing Zones. The goal is to automate this by creating a tool that will automatically calculate IP ranges for us, based on some high-level and easy-to-understand details regarding the future networks.

This notebook demonstrates how to achieve this using the ipmgmt module.

This works in

Binder

Let’s start by installing it:

Install-Module ipmgmt -Scope CurrentUser

And now, we’ll import it:

Import-Module ipmgmt

The ipmgmt module comprises only two cmdlets. The Get-VLSMBreakdown cmdlet breaks down a range into smaller ones, making it possible to segment a range into VNETs and then each VNET into subnets. The Get-IPRanges cmdlet, given a list of in-use ranges and a “root” range, attempts to find a free slot of the specified size, which can be handy to prevent IP space wastage.

Get-Command -Module ipmgmt

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Function        Get-IPRanges                                       0.1.5      ipmgmt
Function        Get-VLSMBreakdown                                  0.1.5      ipmgmt

Let’s breakdown our large “root” IP range into smaller ones. To do this, we need to prepare a list of smaller sub-ranges in the form of PowerShell hashtables, like so: @{type = "VNET-HUB"; size = (256-2)}. Here, we specify that the name of the range is VNET-HUB, and the size is 256-2, which is the maximum number of IPs in a /24 subnet, minus 2 for the first and the last IP.

If more than one subnet is required, we create an array of these hashtables:

$subnets = @{type = "VNET-HUB"; size = (256-2)},
           @{type = "VNET-A"; size = (256-2)}

Now we can attempt to break down the “root” network:

Get-VLSMBreakdown -Network 10.172.0.0/16 -SubnetSize $subnets | ft type, network, netmask, *usable, cidr -AutoSize

type     Network      Netmask       FirstUsable  LastUsable     Usable Cidr
----     -------      -------       -----------  ----------     ------ ----
VNET-A   10.172.1.0   255.255.255.0 10.172.1.1   10.172.1.254      254   24
VNET-HUB 10.172.0.0   255.255.255.0 10.172.0.1   10.172.0.254      254   24
reserved 10.172.128.0 255.255.128.0 10.172.128.1 10.172.255.254  32766   17
reserved 10.172.64.0  255.255.192.0 10.172.64.1  10.172.127.254  16382   18
reserved 10.172.32.0  255.255.224.0 10.172.32.1  10.172.63.254    8190   19
reserved 10.172.16.0  255.255.240.0 10.172.16.1  10.172.31.254    4094   20
reserved 10.172.8.0   255.255.248.0 10.172.8.1   10.172.15.254    2046   21
reserved 10.172.4.0   255.255.252.0 10.172.4.1   10.172.7.254     1022   22
reserved 10.172.2.0   255.255.254.0 10.172.2.1   10.172.3.254      510   23

Here, we obtained two ranges named VNET-A and VNET-HUB. However, by doing so, we generated some unused slots in the root range. These are marked as reserved for convenience, illustrating what happens to the range when it’s broken down. The smaller the sub-ranges you create, the more of such unused ranges you’ll have in the end.

We can also achieve this using CIDR notation:

$subnets = @{type = "GTWSUBNET"; cidr = 27},
@{type = "DMZSUBNET"; cidr = 26},
@{type = "EDGSUBNET"; cidr = 27},
@{type = "APPSUBNET"; cidr = 26},
@{type = "CRESUBNET"; cidr = 26}

Get-VLSMBreakdown -Network 10.10.5.0/24 -SubnetSizeCidr $subnets | ft -AutoSize

type      Network     AddressFamily Netmask         Broadcast   FirstUsable LastUsable  Usable Total
----      -------     ------------- -------         ---------   ----------- ----------  ------ -----
EDGSUBNET 10.10.5.224  InterNetwork 255.255.255.224 10.10.5.255 10.10.5.225 10.10.5.254     30   32
GTWSUBNET 10.10.5.192  InterNetwork 255.255.255.224 10.10.5.223 10.10.5.193 10.10.5.222     30   32
CRESUBNET 10.10.5.128  InterNetwork 255.255.255.192 10.10.5.191 10.10.5.129 10.10.5.190     62   64
APPSUBNET 10.10.5.64   InterNetwork 255.255.255.192 10.10.5.127 10.10.5.65  10.10.5.126     62   64
DMZSUBNET 10.10.5.0    InterNetwork 255.255.255.192 10.10.5.63  10.10.5.1   10.10.5.62      62   64

Now, let’s try to use what we have. To do that, we need to authenticate to Azure. When running locally, you can simply do:

Login-AzAccount

However, in Binder, it needs to be slightly different, like so:

Connect-AzAccount -UseDeviceAuthentication

Once authenticated, we can create networks, for example, like this. Here we first filter out the reserved ones for simplicity.

$vnets = Get-VLSMBreakdown -Network 10.172.0.0/16 -SubnetSize $subnets | ? type -ne 'reserved'

$vnets | % {
    New-AzVirtualNetwork -Name  $_.type -ResourceGroupName 'vnet-test' `
                         -Location 'eastus2' -AddressPrefix "$($_.Network)/$($_.cidr)" | select name, AddressSpace, ResourceGroupName, Location
}

Now, assume at some point we need to add a few more networks. Simultaneously, we might want to reuse one of the reserved slots if it matches the size. This is what Get-IPRanges does. It takes a list of IP ranges “in-use” and returns slots that can fit the range in question. For instance, in our case, we have a “base range” of 10.10.0.0/16 and two ranges in-use 10.10.5.0/24, 10.10.7.0/24. We are looking for a range of size /22. So, the cmdlet recommends us to use the 10.172.4.0/22, which is one of the reserved ranges from the previous example.

Get-IPRanges -Networks "10.172.1.0/24", "10.172.0.0/24" -CIDR 22 -BaseNet "10.172.0.0/16" | ft -AutoSize

IsFree Network    AddressFamily Netmask       Broadcast    FirstUsable LastUsable   Usable Total Cidr
------ -------    ------------- -------       ---------    ----------- ----------   ------ ----- --
False  10.172.0.0  InterNetwork 255.255.255.0 10.172.0.255 10.172.0.1  10.172.0.254    254   256 24
False  10.172.1.0  InterNetwork 255.255.255.0 10.172.1.255 10.172.1.1  10.172.1.254    254   256 24
True   10.172.4.0  InterNetwork 255.255.252.0 10.172.7.255 10.172.4.1  10.172.7.254   1022  1024 22

What if we need to find more than just one range at a time? No worries. We can accomplish this with the following script. Here, we are using Azure as the source of truth, as it allows us to always query it for the real IP ranges that are in use.

So, the steps to achieve this are quite straightforward:

  1. Create a list of sizes we want to create and store it in a variable - $cidrRange.
  2. Pull the ranges from Azure, assuming they are in use by someone - $existingRanges.
  3. Cast whatever we pulled from Azure to System.Net.IPNetwork for correctness. This type is used inside the ipmgmt module to store information about networks and perform all the calculations, comparisons, etc.
  4. Now we run through the list of sizes, for each of them ask Get-IPRanges to find a proper slot, and accumulate the results.

Now, we just need to mark the new ranges as free, to see what we’ve got. For that, we compare what we have in Azure to what we just calculated, and mark the difference accordingly.

$cidrRange = 25,25,24,24,24,24,23,25,26,26 | sort
$existingRanges = (Get-AzVirtualNetwork -ResourceGroupName vnet-test | 
    select name, @{l = "AddressSpace"; e = { $_.AddressSpace.AddressPrefixes }}, ResourceGroupName, Location |
    select -expand AddressSpace)
$existingNetworks = $existingRanges | % {[System.Net.IPNetwork]$_}
$nets = $existingRanges

$ret = @()

$cidrRange | % {
    $ret = Get-IPRanges -Networks $nets -CIDR $_ -BaseNet "10.172.0.0/16"
    $nets = ($ret | select @{l="range"; e = {"$($_.network)/$($_.cidr)"}}).range
}

$ret | % {
    if ( -not ($_ -in $existingNetworks)) {$_.IsFree = $true}
}

$ret | ft -AutoSize

IsFree Network      AddressFamily Netmask         Broadcast    FirstUsable  LastUsable   Usable Total
------ -------      ------------- -------         ---------    -----------  ----------   ------ ---
False 10.172.0.0    InterNetwork 255.255.255.0   10.172.0.255 10.172.0.1   10.172.0.254    254 256
False 10.172.1.0    InterNetwork 255.255.255.0   10.172.1.255 10.172.1.1   10.172.1.254    254 256
True  10.172.2.0    InterNetwork 255.255.254.0   10.172.3.255 10.172.2.1   10.172.3.254    510 512
True  10.172.4.0    InterNetwork 255.255.255.0   10.172.4.255 10.172.4.1   10.172.4.254    254 256
True  10.172.5.0    InterNetwork 255.255.255.0   10.172.5.255 10.172.5.1   10.172.5.254    254 256
True  10.172.6.0    InterNetwork 255.255.255.0   10.172.6.255 10.172.6.1   10.172.6.254    254 256
True  10.172.7.0    InterNetwork 255.255.255.0   10.172.7.255 10.172.7.1   10.172.7.254    254 256
True  10.172.8.0    InterNetwork 255.255.255.128 10.172.8.127 10.172.8.1   10.172.8.126    126 128
True  10.172.8.128  InterNetwork 255.255.255.128 10.172.8.255 10.172.8.129 10.172.8.254    126 128
True  10.172.9.0    InterNetwork 255.255.255.128 10.172.9.127 10.172.9.1   10.172.9.126    126 128
True  10.172.9.128  InterNetwork 255.255.255.192 10.172.9.191 10.172.9.129 10.172.9.190     62  64
True  10.172.9.192  InterNetwork 255.255.255.192 10.172.9.255 10.172.9.193 10.172.9.254     62  64

This provides us with all the necessary information to add a few more networks to our Azure environment.

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