Generate IP range from CIDR notation issue in powershell

Question

I have the following powershell function, and I want to return the IP range from a CIDR.

Here is the code:

function ConvertFrom-CidrNotation {
    param(
        [Parameter(Mandatory=$true)]
        [string]$cidr
    )

    $parts = $cidr -split "/"
    $ip = $parts[0]
    $prefix = $parts[1]

    $subnet = [math]::pow(2, (32 - $prefix))
    $startIP = ([System.Net.IPAddress]::Parse($ip).GetAddressBytes() | ForEach-Object{ $_ -bor 0 }) -join "."
    
    # Correct the end IP calculation
    $ipBytes = [System.Net.IPAddress]::Parse($ip).GetAddressBytes()
    $subnetBytes = [BitConverter]::GetBytes([int]$subnet)
    $endIPBytes = for ($i = 0; $i -lt 4; $i++) { $ipBytes[$i] -bor $subnetBytes[$i] }
    $endIP = $endIPBytes -join "."

    return "$startIP - $endIP"
}

Please, can you tell me why I get wrong results?

Please explain "wrong result". What is the expected result? How do you test? — harper
– harper, Commented Nov 22, 2023 at 13:18
Please show at least one CIDR address, the results this code produced, and the desired results. MRE — lit
– lit, Commented Nov 22, 2023 at 14:27
I'm afraid you wont get out of this without going through binary notation as based on your prefix it is uncertain that lower boundary is the prefix or below. E.g. 192.168.1.0/23 goes from 192.168.0.0 to 192.168.1.255 (including network and broadcast. — Civette
– Civette, Commented Nov 22, 2023 at 14:40
This one works: gist.github.com/davidjenni/7eb707e60316cdd97549b37ca95fbe93 — js2010
– js2010, Commented Nov 22, 2023 at 16:07

jdweng · Accepted Answer · 2023-11-22 18:09:25Z

1

Use following to get start address

$ip = '192.3.6.8' -split '\.'

$startIP = 0
for($i = 3; $i -ge 0; $i--)
{
   $startIP += ([int]::Parse($ip[3-$i])) -shl (8 * $i)
}

Write-Host $startIP.ToString("x4")

answered Nov 22, 2023 at 18:09

jdweng

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