04 / RELIABLE NETWORK AUTOMATION · LESSON 13

Package reusable network helpers

Expose one stable subnet command from a local script module while keeping its converter private.

12 min · learn at your pace

Give callers a deliberate public command

The supplied SubnetTools.ps1 contains the converter and membership function you have already used. Package those helpers in NetworkTools.psm1, exporting only Test-IPv4Subnet. The numeric converter is an implementation detail and does not need to become part of the module's public interface.

Dot-source the helper from inside the module using $PSScriptRoot, which refers to the executing script's directory. This keeps the helper path independent of the caller's current working directory.

Import-Module '/home/user/Example.psm1' -Force
Get-NetworkLabLabel

Test the saved module, not only the editor session

A function defined in main.ps1 is not a saved reusable module. The checks import NetworkTools.psm1 and inspect its exported commands, then exercise boundary and invalid-input behaviour through that module.

Write module source from the editor with a single-quoted here-string so $PSScriptRoot is evaluated when the module runs, rather than while constructing the string. Use Export-ModuleMember -Function Test-IPv4Subnet to declare the public surface explicitly.

The helper must retain the previous contract: /0, /31, and /32 work; first and last addresses are members; malformed inputs throw. This local module needs no installation from the PowerShell Gallery and performs no network operations.

Go deeper

Read the official reference.

Try an example.ps1

Import-Module '/home/user/Example.psm1' -Force
Get-NetworkLabLabel

YOUR TURN

Put it into practice.

Create /home/user/NetworkTools.psm1. Load the supplied SubnetTools.ps1 from the module directory and export only Test-IPv4Subnet. Keep its existing membership and validation contract. The checker imports your saved module from a different working directory; defining commands only in main.ps1 is insufficient.

See an explained solution

One way to solve it.ps1

@'
. "$PSScriptRoot/SubnetTools.ps1"
Export-ModuleMember -Function Test-IPv4Subnet
'@ | Set-Content '/home/user/NetworkTools.psm1'
Import-Module '/home/user/NetworkTools.psm1' -Force

The module resolves its helper relative to its own file and exports one intentional public function. Callers can reuse the subnet contract without depending on the converter implementation.

function ConvertTo-IPv4Number { param([Parameter(Mandatory)][AllowEmptyString()][string]$Address) if ($Address -cnotmatch '\A(?:0|[1-9][0-9]{0,2})(?:\.(?:0|[1-9][0-9]{0,2})){3}\z') { throw 'Expected four canonical decimal octets.' } [uint64]$value = 0 foreach ($part in $Address.Split('.')) { $octet = [int]$part if ($octet -gt 255) { throw 'Octet outside 0 to 255.' } $value = $value * 256 + $octet } return $value } function Test-IPv4Subnet { param([string]$Address,[string]$Cidr) $parts = $Cidr.Split('/') if ($parts.Count -ne 2 -or $parts[1] -cnotmatch '\A(?:0|[1-9][0-9]?)\z' -or [int]$parts[1] -gt 32) { throw 'Invalid IPv4 prefix length.' } $addressValue = ConvertTo-IPv4Number $Address $prefixValue = ConvertTo-IPv4Number $parts[0] [uint64]$blockSize = [math]::Pow(2, 32 - [int]$parts[1]) return [bool]([math]::Floor($addressValue / $blockSize) -eq [math]::Floor($prefixValue / $blockSize)) }