vb For loop repeat a certain number of times - vb.net

Question

So I am making a port scanner and have a min and max port, but can't get the port scanner to stop scanning when it reaches the maximum port?
I have tried doing an Exit For when port reaches portmax.
Here is the code:
Public Class Form1
Public Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim counter As Integer
Button2.Enabled = False
'set counter explained before to 0
counter = 0
End Sub
Private Sub Timer1_Tick(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Timer1.Tick
Dim host As String
Dim counter As Integer
Dim portmin As Integer = TextBox3.Text
Dim portmax As Integer = TextBox2.Text
'Set the host and port and counter
counter = counter + 1 'counter is for the timer
host = TextBox1.Text
For port As Integer = portmin To portmax
' Next part creates a socket to try and connect
' on with the given user information.
Dim hostadd As System.Net.IPAddress = _
System.Net.Dns.GetHostEntry(host).AddressList(0)
Dim EPhost As New System.Net.IPEndPoint(hostadd, port)
Dim s As New System.Net.Sockets.Socket( _
System.Net.Sockets.AddressFamily.InterNetwork, _
System.Net.Sockets.SocketType.Stream, _
System.Net.Sockets.ProtocolType.Tcp)
Try
s.Connect(EPhost)
Catch
End Try
If Not s.Connected Then
ListBox1.Items.Add("Port " + port.ToString + " is not open")
Else
ListBox1.Items.Add("Port " + port.ToString + " is open")
ListBox2.Items.Add(port.ToString)
End If
Label3.Text = "Open Ports: " + ListBox2.Items.Count.ToString
Next
End Sub
Private Sub Button2_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button2.Click
'stop button
Timer1.Stop()
Timer1.Enabled = False
Button1.Enabled = True
Button2.Enabled = False
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles Button1.Click
ListBox1.Items.Add("Scanning: " + TextBox1.Text)
ListBox1.Items.Add("-------------------")
Button2.Enabled = True
Button1.Enabled = False
Timer1.Enabled = True
Timer1.Start()
End Sub
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
End Sub
Private Sub TextBox2_TextChanged(sender As Object, e As EventArgs) Handles TextBox2.TextChanged
End Sub
Private Sub ListBox2_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ListBox2.SelectedIndexChanged
End Sub
End Class
I would really appreciate any help
thanks,

Answer 1

Try to stop the Timer while in the scanning.
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
Timer1.Enabled = False
'Your code
Timer1.Enabled = True
End Sub
If thats the problem, and looks like it, you should consider using a Try/Catch block:
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
Try
Timer1.Enabled = False
'Your code
Catch ex As Exception
'Manage the error
Finally
Timer1.Enabled = True
End Try
End Sub

Answer 2

Try this
For port As Integer = portmin To portmax - 1