How to check user is logged in or not

Hi All,
I have one application for collection centres in the city in which ADMIN will have access to all pages in it.
In which I have added functionality for admin to see Online users/offline users collection and there collection center name.
How can i see the users online automatically when they will be logged in on application.
Like we all see in google talk, yahoo messanger, etc like that only
I also want to keep the option like whether to view only online user or offline users etc.
I have tried the following code for getting the Ip address for the computer..
But I am unable find how user should be shown as active
ip=Request.ServerVariables("HTTP_X_FORWARDED_FOR") ;
if (!string.IsNullOrEmpty(ip))
{
    string[] ipRange = ip.Split(',');
    int le = ipRange.Length - 1;
    string trueIP = ipRange[le];
}
Else
{
ip=Request.ServerVariables ...