Session for Ecommerce Website

what will be userid, when user is not login in his account or if there is no process of Login.and user can buy his products direct way or without login.
Can i use UserIP address for his Userid otherwise any other best solution you have ?

even in the above case you need to provide some custom id to the user for the current session, i.e if the user closes his/her browser you need to clear the session...post some code as to what you have tried..then may be we will get a clear picture...

I am using USer Machine Ip Address so, i create a User ip session in Global.asax file
my code is

void Session_Start(object sender, EventArgs e)
{
// Code that runs when a new session is started

HttpRequest request = base.Request;

string address = request.UserHostAddress;

string result = string.Empty;
string ip = HttpContext.Current.Request.ServerVariables["HTTP_X_FORWARDED_FOR"];
if (!string.IsNullOrEmpty(ip))
{
string[] ipRange = ip.Split(',');
int le = ipRange.Length - 1;
result = ipRange[0];
}
else
{
result = HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"];
}


Session["client_ip"] = result.ToString();

}
when a user use "Add To Cart" then i also pass the "Client_IP" string for his Identification.
Please give me any other best solution and tell me how i can clear session when then use close his browser.