Public Function GetIPChecksum(ByVal Data As String) As Integer
Dim Checksum As Integer, ToAdd As Integer, n As Long
Dim sh As Long
Dim b1 As Integer, b2 As Integer
 
'Summe der 16-bit Words
For n = 1 To Len(Data) Step 2
    b1 = Asc(Mid$(Data, n, 1))
    sh = ShiftL(Asc(Mid$(Data, n + 1, 1)), 8)
    If sh < 32768 Then b2 = sh Else b2 = 32768 - sh
    ToAdd = b2 Or b1
    Checksum = Checksum + ToAdd
    If ToAdd > Checksum Then Checksum = Checksum + 1
Next n
 
If Len(Data) And &H1 Then
    ToAdd = Asc(Mid$(Data, Len(Data) - 1, 1))
    Checksum = Checksum + ToAdd
    If ToAdd > Checksum Then Checksum = Checksum + 1
End If
 
GetIPChecksum = Checksum
 
End Function