'place this code in a separate module

Const shBits As Boolean = True 'True = show debug with bits using showBits function
Public Function ipRange(ByVal ipAddr As String, _
                        ByVal ipMask As String) As List(Of String)
    'returns a list of all ip addresse on a given network
    '
    'ipRange("192.168.3.133", "255.255.254.0") '512 hosts - octet crossing
    '
    '255.255.254.0 is /23
    'List(Of String) - partial output
    '192.168.2.0
    '192.168.2.1
    '192.168.2.2
    '...
    '192.168.2.254
    '192.168.2.255
    '192.168.3.0
    '192.168.3.1
    '...
    '192.168.3.253
    '192.168.3.254
    '192.168.3.255
    'note
    'first entry in list is THE network number
    'last entry in list is the directed broadcast
    '
    nMSK = IPasNum(ipMask) 'get the Mask as number
    CIDR = nCIDR 'get CIDR
    nIP = IPasNum(ipAddr) 'get the IP address as number
    nNET = IPnetAsNum(nIP, nMSK) 'get THE network number as number
    addrs = nNET 'addrs = THE network number as number
    nHOST = IPnumOfHosts(nMSK) 'get the number of hosts

    If shBits Then
        Debug.WriteLine("")
        Debug.WriteLine("IP".PadRight(10, " "c) & showBits(nIP) & " " & ipAddr)
        Debug.WriteLine("Mask".PadRight(10, " "c) & showBits(nMSK) & " " & ipMask)
        Debug.WriteLine("Net".PadRight(10, " "c) & showBits(nNET) & " " & NumToIp(nNET))
        Debug.WriteLine("Hosts".PadRight(10, " "c) & showBits(nHOST) & " " & nHOST.ToString)
        Debug.WriteLine("CIDR".PadRight(10, " "c) & CIDR.ToString)
        'Stop
    End If

    If nIP < 0 OrElse nMSK < 0 OrElse nNET = 0 OrElse nHOST = 0 Then
        Return New List(Of String) 'error
    End If
    ipRange = New List(Of String)
    For x As Long = 0 To nHOST
        ipRange.Add(NumToIp(addrs)) 'add host to list
        addrs += 1 'increment it
    Next
End Function

Public Function IPnets(ByVal ipAddr As String, _
                       ByVal curCIDR As Integer, _
                       ByVal newCIDR As Integer) As List(Of String)
    'returns a list of sub-nets using any IP in the current network,
    'the current mask as CIDR and new mask as CIDR.
    '192.168.3.? /23 to /25 - octet crossing
    '
    'IPnets("192.168.3.128", 23, 25)
    '
    'List(Of String)
    '255.255.255.128
    '192.168.2.0
    '192.168.2.128
    '192.168.3.0
    '192.168.3.128
    '
    Dim curMASK, newMASK, netPlus1 As Long, numOfNets As Integer
    nIP = IPasNum(ipAddr) 'get the IP address as number
    If nIP < 0 OrElse curCIDR < 2 OrElse curCIDR > newCIDR OrElse newCIDR > 30 Then
        Return New List(Of String) 'error
    End If
    IPnets = New List(Of String)
    curMASK = MaskFromCidr(curCIDR) 'convert CIDR's to mask as number
    newMASK = MaskFromCidr(newCIDR)
    nHOST = IPnumOfHosts(newMASK)
    netPlus1 = nHOST + 1 'used to add 1 to the network portion of the ip address
    nNET = IPnetAsNum(nIP, curMASK) 'get THE network number as number
    numOfNets = CInt(2 ^ (newCIDR - curCIDR)) 'number of networks
    IPnets.Add(NumToIp(newMASK)) '<<<<<<<<<<<<<<<<<<<<< first entry in list is the new mask

    If shBits Then
        Debug.WriteLine("")
        Debug.WriteLine("IP".PadRight(10, " "c) & showBits(nIP) & " " & ipAddr)
        Debug.WriteLine("Network".PadRight(10, " "c) & showBits(nNET) & " " & NumToIp(nNET))
        Debug.WriteLine("Cur. Mask".PadRight(10, " "c) & showBits(curMASK) & " " & NumToIp(curMASK))
        Debug.WriteLine("New Mask".PadRight(10, " "c) & showBits(newMASK) & " " & NumToIp(newMASK))
        Debug.WriteLine("Plus 1".PadRight(10, " "c) & showBits(netPlus1) & " " & NumToIp(netPlus1))
        Debug.WriteLine("Num. Nets".PadRight(10, " "c) & numOfNets.ToString)
        'Stop
    End If

    For x As Integer = 0 To numOfNets - 1
        IPnets.Add(NumToIp(nNET))
        'to get last address(broadcast) Dim BrdCst as long = nNET Or nHOST
        nNET += netPlus1 'add one network number
    Next
End Function

Private Function MaskFromCidr(ByVal CIDR As Integer) As Long
    'x = 32 - CIDR
    'z = (2^x)-1
    'return z xor 255.255.255.255
    MaskFromCidr = CLng(2 ^ ((32 - CIDR)) - 1) Xor IPbrdcst
End Function

Private Function IPnumOfHosts(ByVal IPmsk As Long) As Long 'a mask for the host portion
    '255.255.255.0 XOR 255.255.255.255 = 255 so 0 to 255 is 256 hosts
    IPnumOfHosts = IPmsk Xor IPbrdcst 'cacluate the number of hosts
End Function

Private Function IPnetAsNum(ByVal IPnum As Long, ByVal IPmsk As Long) As Long 'extract network from ip address
    '192.168.2.113 AND 255.255.255.0 = 192.168.2.0
    IPnetAsNum = IPnum And IPmsk 'calculate THE network number
End Function

Private Function IPasNum(ByVal AipAddr As String) As Long 'return -1 on error
    If Not IPAddress.TryParse(AipAddr, aIP) Then 'convert string to IP
        Return -1 'error
    Else
        IPasNum = IPtoNum(aIP) 'convert IP to number
        If IPasNum = 0 Then
            Return -1 'error
        End If
    End If
End Function
Private Function IPtoNum(ByVal theIP As IPAddress) As Long 'convert IP to number
    Dim IPb() As Byte = theIP.GetAddressBytes 'get the octets
    Dim theBit As Integer = 31 'work MSb to LSb
    Dim addr As Long 'accumulator for address
    nCIDR = 0
    For x = 0 To 3 'four octets
        For y = 7 To 0 Step -1 'with 8 bits each - duh
            If (IPb(x) And CByte(2 ^ y)) = CByte(2 ^ y) Then 'if the bit is on
                addr += CLng(2 ^ theBit) 'accumulate
                nCIDR += 1 'count bits
            End If
            theBit -= 1 'decrement bit pos
        Next
    Next
    Return addr
End Function
Private Function NumToIp(ByVal theIP As Long) As String 'convert number back to IP
    Dim IPb(3) As Byte '4 octets
    Dim theBit As Integer = 31 'work MSb to LSb
    Dim addr As String 'accumulator for address
    For x = 0 To 3 'four octets
        For y = 7 To 0 Step -1 '8 bits
            If (theIP And CLng(2 ^ theBit)) = CLng(2 ^ theBit) Then 'if the bit is on
                IPb(x) += CByte(2 ^ y) 'accumulate
            End If
            theBit -= 1
        Next
        addr &= IPb(x).ToString & "." 'add current octet to string
    Next
    Return addr.TrimEnd("."c)
End Function
Private Function showBits(ByVal aNum As Long) As String
    Dim strObits As New StringBuilder
    strObits.Append(Convert.ToString(aNum, 2).PadLeft(32, "0"c))
    For ix As Integer = 24 To 0 Step -8
        strObits.Insert(ix, " ")
    Next
    showBits = strObits.ToString.Trim
End Function