Article ID: 194938 - Last Review: July 2, 2004 - Revision: 1.2

How To Monitor TCP/IP Ports in Use

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SUMMARY

This article describes how to monitor whether a TCP/IP port is in use.

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MORE INFORMATION

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.

Create a new Standard EXE project. Form1 is created by default.
Add 1 Timer, 2 Labels, 1 Listbox, 1 TextBox and 2 CommandButton controls to Form1. Position Label1 above Text1 and Label2 above List1.
Set the Sorted property of List1 to True.

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

         List1.Clear
         For X = 1 To PortsChecked
            DoEvents
            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

            WinSock1.Close
         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

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.

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