Converts between PrefixLength and subnet mask.

Format-Prefix.ps1

#obatined from: https://gallery.technet.microsoft.com/scriptcenter/Convert-SubnetMask-80401493
<#
.SYNOPSIS
Converts between PrefixLength and subnet mask. 
.DESCRIPTION
This script converts between PrefixLength and subnet mask parameters, these parameters define the size of a subnet for IPv4 addresses.  
This script assumes valid subnet mask input and does not support scenarios such as non-contiguous subnet masks. 
.INPUTS
None
.OUTPUTS
The script outputs a PrefixLength if SubnetMask was entered, or a SubnetMask if a PrefixLength was entered.
.NOTES
Requires Windows 8 or later.
#>

function Format-Prefix{
param( 

    [Parameter(ParameterSetName="SubnetMask",Mandatory=$True)][string]$SubnetMask, 
    [Parameter(ParameterSetName="PrefixLength",Mandatory=$True)][int]$PrefixLength)


####################################
#User provided a prefix
if ($PrefixLength)
{
    $PrefixLengthExit = $PrefixLength
    if ($PrefixLength -gt 32) 
    { 
        write-host "Invalid input, prefix length must be less than 32"
        exit(1)
    }
               
    $bitArray=""
    for($bitCount = 0; $PrefixLength -ne "0"; $bitCount++) 
    {
        $bitArray += '1'
        $PrefixLength = $PrefixLength - 1;
    }
    
    ####################################                       
    #Fill in the rest with zeroes
    While ($bitCount -ne 32) 
    {
        $bitArray += '0'
        $bitCount++ 
    }
    ####################################
    #Convert the bit array into subnet mask
    $ClassAAddress = $bitArray.SubString(0,8)
    $ClassAAddress = [Convert]::ToUInt32($ClassAAddress, 2)
    $ClassBAddress = $bitArray.SubString(8,8)
    $ClassBAddress = [Convert]::ToUInt32($ClassBAddress, 2)
    $ClassCAddress = $bitArray.SubString(16,8)
    $ClassCAddress = [Convert]::ToUInt32($ClassCAddress, 2)
    $ClassDAddress = $bitArray.SubString(24,8)           
    $ClassDAddress = [Convert]::ToUInt32($ClassDAddress, 2)
 
    $SubnetMaskExit =  "$ClassAAddress.$ClassBAddress.$ClassCAddress.$ClassDAddress"
}

####################################
##User provided a subnet mask
if ($SubnetMask)
{
	####################################
    #Ensure valid IP address input.  Note this does not check for non-contiguous subnet masks!
    $Address=[System.Net.IPaddress]"0.0.0.0"
    Try
    {
        $IsValidInput=[System.Net.IPaddress]::TryParse($SubnetMask, [ref]$Address)
    }
    Catch 
    {

    }
    Finally
    {

    }    

    if ($IsValidInput -eq $False)
    {
        Write-Host "Invalid Input. Please enter a properly formatted subnet mask."
        Exit(1)
    }

    ####################################
    #Convert subnet mask to prefix length
    If($IsValidInput)
    {
        $PrefixArray=@()
        $PrefixLength = 0
        $ByteArray = $SubnetMask.Split(".")
        
        ####################################        
        #This loop converts the bytes to bits, add zeroes when necessary
        for($byteCount = 0; $byteCount-lt 4; $byteCount++) 
        {
            $bitVariable = $ByteArray[$byteCount]
            $bitVariable = [Convert]::ToString($bitVariable, 2)
            
            if($bitVariable.Length -lt 8)
            {
              $NumOnes=$bitVariable.Length
              $NumZeroes=8-$bitVariable.Length

              for($bitCount=0; $bitCount -lt $NumZeroes; $bitCount++) 
              {
                $Temp=$Temp+"0"
              }
              
              $bitVariable=$Temp+$bitVariable
            }
            
            ####################################
            #This loop counts the bits in the prefix
            for($bitCount=0; $bitCount -lt 8; $bitCount++) 
            {
                if ($bitVariable[$bitCount] -eq "1")
                {
                    $PrefixLength++ 
                }

                $PrefixArray=$PrefixArray + ($bitVariable[$bitCount])

            }
        }
        
        ####################################
        #Check if the subnet mask was contiguous, fail if it wasn't.
        $Mark=$False

        foreach ($bit in $PrefixArray) 
        {
            if($bit -eq "0")
            {
                if($Mark -eq $False)
                {
                    $Mark=$True
                }
            }
            if($bit -eq "1")
            {
                if($Mark -eq $True)
                {
                    Write-Host "Invalid Input. Please enter a properly formatted subnet mask."
                    Exit(1)
                }    
            }
                
        }

	    $SubnetMaskExit = $SubnetMask
	    $PrefixLengthExit = $PrefixLength
	}
}
##Create the object to be Exited to the console
$Exit = new-object Object
Add-Member -InputObject $Exit -Name PrefixLength -Value $PrefixLengthExit -Type NoteProperty
Add-Member -InputObject $Exit -Name SubnetMask -Value  $SubnetMaskExit -Type NoteProperty
$Exit
}