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.
- 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:
Const PortsChecked = 200
Private Sub Command1_Click()
Timer1.Enabled = True
Timer1.Interval = 1000
Private Sub Command2_Click()
Timer1.Interval = 0
Timer1.Enabled = False
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
Private Sub Form_Load()
Label1.Caption = "Checking Port #"
Label2.Caption = "Ports In Use"
Command1.Caption = "Start"
Command2.Caption = "End"
Text1.Locked = True
- 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
For additional information, please see the following article in the
Microsoft Knowledge Base:
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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