Make a userform with a textbox named txtIP, a command button named
cmdClass, and a label named lblClass (plus any other controls you
like) and give the button this code:

Private Sub cmdClass_Click()
   Dim IPaddr As Variant
   
   If Len(txtIP.Text) > 0 Then
       IPaddr = Split(txtIP.Text, ".")
       If UBound(IPaddr) <> 3 Then
           lblClass = "Not a valid IP address"
           Exit Sub
       End If
       
       Select Case (CLng(IPaddr(0)))
           Case 1 To 126
               lblClass = "Class A"
           Case 128 To 191
               lblClass = "Class B"
           Case 192 To 223
               lblClass = "Class C"
           Case Else
               lblClass = "Not classified"
       End Select
   End If
End Sub

The Split function makes an array in the Variant, consisting of the
nodes of the address separated by periods (the periods aren't stored
in the Variant). The Select Case then figures out which range the
first node belongs to, and assigns the corresponding value to the
label.

The check for the upper bound of the variant makes sure there are four
nodes (numbering starts at 0). You could also throw in checks to make
sure all the nodes are numeric and that none of them have values
greater than 255; failing either test would also make the IP address
invalid.

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org
Email cannot be acknowledged; please post all follow-ups to the
newsgroup so all may benefit.

Call_me_sol,
Greg Maxey showed you last month how get your data and work with select case
with the macro below.
So what are you having a problem with now?
You can use the left() function to return the first three characters.

Private Sub CommandButton1_Click()
Dim myArray
Dim lngCount As Long
Dim i As Long
Dim pString As String
Dim pString2
myArray = Split(Me.TextBox1.Text, Chr(13))
lngCount = UBound(myArray) + 1
Select Case lngCount
 Case Is < 10
   pString2 = "ip prefix maximum 100"
 Case Is > 11
   Select Case lngCount
     Case Is < 100
       pString2 = "ip prefix list maximum 1000"
     Case Is > 100
       pString2 = "ip prefix list maximum 10000"
     Case Else
   End Select
 Case Else
   pString = "Whatever"
End Select
For i = 0 To UBound(myArray)
 pString = myArray(i)
 ActiveDocument.Range.InsertAfter "ip prefix list AS2345 permit ip "
& pString & vbCr
Next i
 ActiveDocument.Range.InsertAfter pString2
Me.Hide
End Sub