subnet_checker.ps1

# This script uses this library: https://github.com/lduchosal/ipnetwork
# It needs to be imported using e.g. `Add-Type -AssemblyName ./System.Net.IPNetwork.dll`
# Not to be confused with IPNetwork from Microsoft.AspNetCore.HttpOverrides.dll
function Get-CIDRFromAddressByPrefixList {
    [OutputType([System.Net.IPNetwork])]
    param(
        [Parameter(Mandatory)][System.Net.IPAddress]$address,
        [Parameter(Mandatory)][System.Net.IPNetwork[]]$prefixes
    )
    $address6 = $address.MapToIPv6()
    if ($address6.IsIPv4MappedToIPv6) { $address = $address6.MapToIPv4() }
    $netmask128 = [System.Net.IPAddress]"ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff"
    foreach ($prefix in $prefixes) {
        $network6 = $prefix.Network.MapToIPv6()
        $netmask6 = [System.Net.IPAddress]::new(([BigInt]$prefix.Netmask.MapToIPv6().GetAddressBytes() + [BigInt]([System.Net.IPAddress]"ffff:ffff:ffff:ffff:ffff::").GetAddressBytes()).ToByteArray())
        $prefix6 = [System.Net.IPNetwork]::Parse($network, $netmask)
        if ($prefix6.Overlap( [System.Net.IPNetwork]::Parse($address6, $netmask128) )) {
            $CIDRString = '{0}/{1}' -f $address6, $netmask6
            return $CIDRString
        }
    }
}

function Get-UnmappedAddress {
    [OutputType([String])]
    param(
        [PArameter(Mandatory)][String]$CIDR
    )
    [System.Net.IPAddress]$IPAddress, [int]$CIDRSuffix = $CIDR -split '/'

    if ($IPAddress.IsIPv4MappedToIPv6) {
        $CIDR = '{0}/{1}' -f $IPAddress.MapToIPv4().IPAddressToString, $CIDRSuffix - 96
    } else {
        $CIDR = '{0}/{1}' -f $IPAddress.IPAddressToString, $CIDRSuffix
    }
    return $CIDR
}

function Test-IsIPv4 {
    [OutputType([Boolean])]
    param(
        [Parameter(Mandatory)][System.Net.IPAddress]$IPAddress
    )
    if ($IPAddress.IsIPv4MappedToIPv6 -or ($IPAddress.AddressFamily -eq [System.Net.Sockets.AddressFamily]::InterNetwork)) {
        return $true
    } else {
        return $false
    }
}