Count online person rely ip address and update online person automatic ??

Question

Hello Friend!
My question is i can count online person by rely the ip address of client ??
My code is i have 1 page .aspx to display online person and the global.asax to increase or reduce online person.This is code in .aspx:
protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Response.Write("Person online is:");
                Response.Write(Application["person_online"].ToString());
            }           
        }

And this is .asax
 protected void Application_Start(object sender, EventArgs e)
        {
            Application["person_online"] = 0;
        }

        protected void Session_Start(object sender, EventArgs e)
        {
            Application.Lock();          
            Application["person_online"] = Convert.ToInt32(Application["person_online"]) + 1;
            //Session.Timeout = 1;
            Application.UnLock();
           
        }
 protected void Session_End(object sender, EventArgs e)
        {
            Application.Lock();
            Application["person_online"] =Convert.ToInt32(Application["person_online"])-1;
            Application.UnLock();
        }



So when i run my page by 3 browser(firefox,internet-explorer,chrome) -> it displayed 3 person online when i reload page but i think it exactly is 1 not 3. Someone give me the way to process it's good??

And the next problem is Can i update the person online automatic .I mean when i turn off 1 browser that one of 3 browser ->so the person online automatic reduce 1 needn't press F5 or reload page on other browser(other client)??Someone help me.Thanks