[pwsh] Generate a random IP address in a given CIDR range

random-ip-in-cidr.ps1

function Get-RandomIPAddressInCIDR {
    param (
        [string]$CIDR
    )

    # Split the CIDR into network address and subnet mask
    $networkAddress, $subnetMaskBits = $CIDR -split '/'
    $subnetMaskBits = [int]$subnetMaskBits

    if ($subnetMaskBits -eq 32) {
        # If the subnet mask is /32, return the network address as the only possible IP
        return [IPAddress]::Parse($networkAddress).IPAddressToString
    }

    # Calculate the number of possible host addresses based on the subnet mask
    $possibleHostAddresses = [math]::Pow(2, 32 - $subnetMaskBits) - 2

    # Store the original value of $subnetMaskBits
    $originalSubnetMaskBits = $subnetMaskBits

    do {
        # Restore the original value of $subnetMaskBits
        $subnetMaskBits = $originalSubnetMaskBits

        # Generate a random number within the range of possible host addresses
        $randomHostAddress = Get-Random -Minimum 0 -Maximum ($possibleHostAddresses + 1)

        # Convert the network address to bytes
        $networkBytes = [System.Net.IPAddress]::Parse($networkAddress).GetAddressBytes()

        # Convert the random host address to bytes
        $randomBytes = [BitConverter]::GetBytes($randomHostAddress)

        # Combine the network address bytes with the random host address bytes
        $randomIPBytes = for ($i = 0; $i -lt 4; $i++) {
            if ($subnetMaskBits -ge 8) {
                $subnetMaskBits -= 8
                $networkBytes[$i]
            }
            else {
                $mask = [math]::Pow(2, 8 - $subnetMaskBits) - 1
                $subnetMaskBits = 0
                (($networkBytes[$i] -band (-bnot $mask)) -bor ($randomBytes[$i] -band $mask))
            }
        }

        # Convert the resulting bytes back to an IP address string
        $randomIP = [IPAddress]::Parse($randomIPBytes -join '.')
    } while ($randomIP.GetAddressBytes()[3] -eq 0 -or $randomIP.GetAddressBytes()[3] -eq 1 -or $randomIP.GetAddressBytes()[3] -eq 254 -or $randomIP.GetAddressBytes()[3] -eq 255) #disallow .0 .1 and .255 as the last octet

    return $randomIP.IPAddressToString 
}

# Usage example
$cidr = "192.168.1.0/24"

# One IP
Get-RandomIPAddressInCIDR $cidr


# Loop for 20 Random IPs
for ($i = 0; $i -lt 20; $i++) {
    $randomIP = Get-RandomIPAddressInCIDR $cidr
    $randomIP
}