How To Monitor TCP/IP Ports in Use

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Article ID: 194938 - View products that this article applies to.
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This article describes how to monitor whether a TCP/IP port is in use.


There are times when it is necessary to know or monitor whether a port is in use. The following sample demonstrates how this is done.
  1. Create a new Standard EXE project. Form1 is created by default.
  2. Add 1 Timer, 2 Labels, 1 Listbox, 1 TextBox and 2 CommandButton controls to Form1. Position Label1 above Text1 and Label2 above List1.
  3. Set the Sorted property of List1 to True.
  4. Add the following code to the General Declarations section of Form1:
          Option Explicit
          Const PortsChecked = 200
          Private Sub Command1_Click()
             Timer1.Enabled = True
             Timer1.Interval = 1000
          End Sub
          Private Sub Command2_Click()
             Timer1.Interval = 0
             Timer1.Enabled = False
          End Sub
          Private Sub Timer1_Timer()
             Dim X As Integer
             For X = 1 To PortsChecked
                Text1.Text = X
                WinSock1.LocalPort = X
                On Error Resume Next
                WinSock1.Listen  ' If we get an error, the port is busy.
                If Err.Number = 10048 Then
                   List1.AddItem X  ' Log Active port # to list box.
                   Err.Number = 0
                End If
             Next X
          End Sub
          Private Sub Form_Load()
             Label1.Caption = "Checking Port #"
             Label2.Caption = "Ports In Use"
             Command1.Caption = "Start"
             Command2.Caption = "End"
             Text1.Locked = True
          End Sub
  5. Run the program. If, for example, you have an active DCOM connection, you should see 135 displayed in the ListBox.

    To change the number of ports checked, modify the constant PortsChecked. To change the amount of time between samples modify the constant TimerInt.


For additional information, please see the following article in the Microsoft Knowledge Base:
173619 PRB: Winsock Control Generates Error 10048 - Address in Use
Also search Visual Basic Help for "Using the Winsock Control."


Article ID: 194938 - Last Review: July 2, 2004 - Revision: 1.2
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
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