Calculate Subnet Mask Function
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
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