How does the server identify a browser ?

The first time when a browser request a page, server establishes a session and creates a session Id. Server returns this ID to the browser as part of the 'Response'. This ID is not displayed to the user. Browser will keep this internally.

There are cases where this Session ID can be visible to the users. Visit HomeDepot.com and hit any link from the home page. Then check the URL in the browser. You can see a string like BV_SessionID=@@@@0941891585.1123945660@@@@. This number represents the session id. This particular site sends the session id as a query string in the URL. If you delete this session id from the url, server will treat it as a new session and will create a new session.

But ideally, in a ASP.NET web site, there is no need to make the session id publicly visible. Server can keep it internally and send to server without annoying you.

After the browser gets a session ID, it will send the session ID to the server along with each additional page requests it makes. The webserver can identify the sessions using this Id. For some reason, if the session times out, then this ID will be no longer valid. In such cases, server will create a new session and will treat this request as a new request.