Given the low and high addresses of a subnet range, will return the subnet mask of the range

Get-SubnetMask.ps1

<#
    .DESCRIPTION
        Given the low and high addresses of a subnet range, will return the subnet mask of the range.
        For example, given 10.0.0.64 and 10.0.0.95, will return "10.0.0.64/27"

    .EXAMPLE
        Get-SubnetMask -Low '10.0.0.64' -High '10.0.0.95'
#>
function Get-SubnetMask
{
    param (
        [Parameter(Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]
        $Low,

        [Parameter(Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]
        $High
    )

    # split low and high
    $low_parts = $Low -split '\.'
    $high_parts = $High -split '\.'

    # subtract and bitwise not to get network
    $network = @(0..3 | ForEach-Object { 256 + (-bnot ([byte]$high_parts[$_] - [byte]$low_parts[$_])) })

    # convert the network to binary
    $binary = ((($network | ForEach-Object { [Convert]::ToString($_, 2) }) -join '') -split '0')[0] -join ''
    $binary = $binary + (New-Object String -ArgumentList "0", (32 - $binary.Length))

    # re-calc the network and low address
    $network = @(0..3 | ForEach-Object { [Convert]::ToByte(($binary[($_ * 8)..(($_ * 8) + 7)] -join ''), 2) })
    $Low = @(0..3 | ForEach-Object { [byte]([byte]$network[$_] -band [byte]$low_parts[$_]) }) -join '.'

    # count the 1 bits of the network
    $bits = ($binary.ToCharArray() | ForEach-Object { Invoke-Expression $_ } | Measure-Object -Sum).Sum

    # return the mask
    return "$($Low)/$($bits)"
}