How restrict same user login from different browser location

Dim _objStudentInfo As StudentInfo = New StudentInfo(Txtuserid.Text, Txtpassword.Text)
       Session("objStudentInfo") = _objStudentInfo
       Dim con As SqlConnection
       con = New SqlConnection("server=***;user id=sa;password=***;database=***")
       con.Open()
       Dim cmd As SqlCommand
       Dim rdr As SqlDataReader
       cmd = New SqlCommand("select USRMST_USERLOGID,USRMST_PASS from [SYSTEM].[SYSTEM_USER_LOGIN] where USRMST_USERLOGID='" + Txtuserid.Text + "' And USRMST_PASS='" + Txtpassword.Text + "'", con)
       rdr = cmd.ExecuteReader()
       While rdr.Read()
           If rdr("USRMST_USERLOGID").ToString() = Txtuserid.Text AndAlso rdr("USRMST_PASS").ToString() = Txtpassword.Text Then
               SingleSessionPreparation.CreateAndStoreSessionToken(Txtuserid.Text)
               Dim sKey As String = Txtuserid.Text + Txtpassword.Text
               Dim sUser As String = Convert.ToString(Cache(sKey))
               If sUser Is Nothing OrElse sUser = [String].Empty Then
                   Dim i = HttpContext.Current.User
                   Dim SessTimeOut As New TimeSpan(0, 0, HttpContext.Current.Session.Timeout, 0, 0)
                   HttpContext.Current.Cache.Insert(sKey, sKey, Nothing, DateTime.MaxValue, SessTimeOut, System.Web.Caching.CacheItemPriority.Normal, Nothing)

                   Dim context As HttpContext = HttpContext.Current
                   Dim page As Page = TryCast(context.Handler, Page)
                   HttpContext.Current.User.Identity.Name.Contains(sKey)
                   page.Session("user") = HttpContext.Current.Session.SessionID
                   ''Session("user") = Txtuserid.Text + Txtpassword.Text

                   ' Let them in - redirect to main page, etc.
                   Response.Redirect("Home.aspx?Uid=" + Txtuserid.Text + "")
               Else
                   ' cache item exists, so too bad...
                   lblDisplay.Text = "<Marquee><h1><font color=red>Already Logged IN</font></h1></marquee>"
                   Return
               End If

               lblDisplay.Text = "Invalid UserName and Password Try Again !!!"
           End If
       End While