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## Calculate Subnet Mask Function

Posted On 2012-09-16 by dwirch
Keywords:
Tags: VB6 Miscellaneous VB6 String Handling VB6 Network Windows
Views: 988

Most developers don't have much of a clue about network fundamentals. What may be a simple thing for a network engineer or system administrator may leave a developer simply scratching his or her head.

This function can help those developers out there that need to figure out what to put into the subnet mask field of their network configuration.

Insert the first and last IP address of your range and it will calculate the subnet mask that fits these values with the starting and ending address for this subnet.

Usage:

```Dim MySubnet as String MySubnet = CalculateSubnetMask("192.168.91.254","192.168.90.0")```

And here is the code for the function. Simply paste this into a module, and you should be good to go.

```Function CalculateSubnetMask(HighIP As String, LowIP As String) As String   'Insert the first and last IP address of your range and it will calculate the subnet mask   'that fits these values with the starting and ending address for this subnet.   'get the high numbers   If UBound(Split(HighIP, ".")) = 3 Then     PrevPos = 0     For x = 1 To 4       Pos = InStr(PrevPos + 1, HighIP, ".", 1)       If x = 4 Then Pos = Len(HighIP) + 1         Num = Int(Mid(HighIP, PrevPos + 1, Pos - PrevPos - 1))         PrevPos = Pos                  Select Case x           Case 1             highnumber4 = Num           Case 2             highnumber3 = Num           Case 3             highnumber2 = Num           Case 4             highnumber1 = Num         End Select     Next   End If   'get the Low numbers   If UBound(Split(LowIP, ".")) = 3 Then     PrevPos = 0     For x = 1 To 4       Pos = InStr(PrevPos + 1, LowIP, ".", 1)       If x = 4 Then Pos = Len(LowIP) + 1       Num = Int(Mid(LowIP, PrevPos + 1, Pos - PrevPos - 1))       PrevPos = Pos       Select Case x         Case 1           lownumber4 = Num         Case 2           lownumber3 = Num         Case 3           lownumber2 = Num         Case 4           lownumber1 = Num       End Select     Next   End If   'Set up result variables.   Dim subnetmask4 As Integer   Dim subnetmask3 As Integer   Dim subnetmask2 As Integer   Dim subnetmask1 As Integer   'Figure out the higest bit that changes.   'We do this by first doing a binary xor of the low and high numbers.   subnetmask4 = highnumber4 Xor lownumber4   subnetmask3 = highnumber3 Xor lownumber3   subnetmask2 = highnumber2 Xor lownumber2   subnetmask1 = highnumber1 Xor lownumber1   'Then we round it up to the next most significant digit.   subnetmask4 = roundup(subnetmask4)   subnetmask3 = roundup(subnetmask3)   subnetmask2 = roundup(subnetmask2)   subnetmask1 = roundup(subnetmask1)   'Figure out which set of numbers changes, then set the lower numbers from it to 0.   If (subnetmask4 < 255) Then     subnetmask3 = 0     subnetmask2 = 0     subnetmask1 = 0   End If   If (subnetmask3 < 255) Then     subnetmask2 = 0     subnetmask1 = 0   End If   If (subnetmask2 < 255) Then subnetmask1 = 0   CalculateSubnetMask = subnetmask4 & "." & subnetmask3 & "." & subnetmask2 & "." & subnetmask1 End Function Function roundup(subnetmaskx As Integer) As Integer 'We need to figure out the most significant bit, then set the subnetmaskx to that number.   Select Case subnetmaskx     Case Is = 0       roundup = 255     Case Is < 2       roundup = 254     Case Is < 4       roundup = 252     Case Is < 8       roundup = 248     Case Is < 16       roundup = 240     Case Is < 32       roundup = 224     Case Is < 64       roundup = 192     Case Is < 128       roundup = 128     Case Is < 256       roundup = 0     Case Else       roundup = 0   End Select    End Function ```

### About the Author

dwirch has posted a total of 181 articles.

You can find more information from dwirch by visiting http://www.derekwirch.com.

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