[Source] Port Scanning Utility

Code:
Imports System.Net.Sockets, System.Threading, System.Net



Public Class Form1



    Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll

        Label1.Text = "Threads: " & TrackBar1.Value

    End Sub



    Dim Active As Boolean

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        If Active Then

            Toggle(True)

        Else

            Toggle(False)



            ListBox1.Items.Clear()

            ProgressBar1.Maximum = NumericUpDown2.Value + 1

            ProgressBar1.Minimum = NumericUpDown1.Value

            ProgressBar1.Value = NumericUpDown1.Value



            Try

                Dim Host = Resolve(TextBox1.Text)

                For O = 0 To TrackBar1.Value - 1

                    Dim T As New Thread(AddressOf DoScan)

                    T.Start(New Object() {NumericUpDown1.Value + O, NumericUpDown2.Value, TrackBar1.Value, Host, NumericUpDown3.Value})

                Next

            Catch

            End Try

        End If

    End Sub



    Private Sub Toggle(ByVal b As Boolean)

        NumericUpDown1.Enabled = b

        NumericUpDown2.Enabled = b

        NumericUpDown3.Enabled = b

        TrackBar1.Enabled = b

        Button1.Text = If(b, "Begin", "Cancel")

        Active = Not b

    End Sub



    Private Sub DoScan(ByVal data As Object)

        For I = data(0) To data(1) Step data(2)

            If Not Active Then Exit For

            Output(Scan(data(3), I, data(4)), I)

        Next

    End Sub



    Delegate Sub OD(ByVal open As Boolean, ByVal port As Integer)

    Private Sub Output(ByVal open As Boolean, ByVal port As Integer)

        If InvokeRequired Then

            Invoke(New OD(AddressOf Output), open, port)

        Else

            If open Then ListBox1.Items.Add(port)

            ProgressBar1.Value += 1

            If ProgressBar1.Value = ProgressBar1.Maximum Then Toggle(True)

        End If

    End Sub



    Private Shared Function Resolve(ByVal host As String) As IPAddress

        Return Dns.GetHostEntry(host).AddressList(0)

    End Function



    Private Shared Function Scan(ByVal host As IPAddress, ByVal port As Integer, ByVal timeout As Integer) As Boolean

        Using T As New Socket(2, 1, 6)

            Try

                T.BeginConnect(host, port, Nothing, 0)

                Dim I = Now.AddMilliseconds(timeout)

                While Now < I

                    Thread.Sleep(1)

                    If T.Connected Then Return True

                End While

            Catch

            End Try

        End Using

    End Function

End Class