With PowerShell 3.0 and onwward you can easily convert a SubnetMask in bit form to a “real” dotted 4 octet format:
PS > $MaskLength = 24; PS > [IPAddress] $ip = 0; PS > $ip.Address = ([UInt32]::MaxValue -1) -shl (32 - $MaskLength) -shr (32 - $MaskLength) PS > $ip Address : 16777215 AddressFamily : InterNetwork ScopeId : IsIPv6Multicast : False IsIPv6LinkLocal : False IsIPv6SiteLocal : False IsIPv6Teredo : False IsIPv4MappedToIPv6 : False IPAddressToString : 255.255.255.0
Reverting that operation is a bit trickier, but easily handable as well:
function Convert-IpAddressToMaskLength([string] $dottedIpAddressString)
{
$result = 0;
# ensure we have a valid IP address
[IPAddress] $ip = $dottedIpAddressString;
$octets = $ip.IPAddressToString.Split('.');
foreach($octet in $octets)
{
while(0 -ne $octet)
{
$octet = ($octet -shl 1) -band [byte]::MaxValue
$result++;
}
}
return $result;
}
PS > Convert-IpAddressToMaskLength 255.254.0.0
15
PS > Convert-IpAddressToMaskLength 255.255.128.0
17
UPDATE: be careful when trying to convert IP addresses with leading zeros, as described in this post.
Ronald Rink
I am a senior auditor and consultant at d-fens for business processes and information systems.
d-fens GmbH 
Fyi, revert doesn’t seem to work with anything other than 255. Nice logic though.
Hi Christopher, thanks for your hint – very much appreciated! I corrected the error and it seems to be working correctly now.
Regards, Ronald
I am interested int this…. however, I am a powershell noobie. How do you run this? And do you run it against a file? If so, what format does the IP addressing need to be in?
`Convert-IpAddressToMaskLength` is a PowerShell function, which you can run directly from the console once you have defined it. Defining the function can be either done by simply pasting it into the console or by “dot-sourcing” it from a file. The format of the IP address would be “dotted”, e.g. 255.254.0.0 (like written in the blog post). If you want to convert multiple IP addresses you can put them into a file (one by a line) and then read them with `Get-Content` and pipe them to the method like this: `Get-Content myAddresses.txt | % { Convert-IpAddressToMaskLength $_ }`. For details on “dot-sourcing” etc have a look at StackOverflow.com or similar QA sites.
The original bit of code doesn’t work either. On PS 5.1 the resulting IPAddressToString turns out to be 254.255.255.0 on the example provided.
Try [UInt32]::MaxValue rather than ([UInt32]::MaxValue -1)
That works but only for full 255 values. If you do something like /10 you get 254.3.0.0 but it should be 255.192.0.0
It does only work for 8, 16, 24 and 32 but not for values in between.