Multiple Login Check

Neither way will be particularly reliable, but I'd use session_onstart and session_onend in the global.asa file to add/delete a user from an active users table and the session_onstart to also check if they are already in it with a different remote_addr or remote_host.

You can't reliably depend upon session_onend to execute so you'd also need something in the on_start event to check for time - e.g. if the user comes back on a different IP but it's been more than 20 minutes, delete the first entry and add him as new.

Yes. You'd need memory space that's available across all sessions so you'd use Application level variables to do this. Probably by writing/deleting from an array of users established in the global.asa Application_OnStart subroutine. boRg uses Application level variables for this forum. If I recall correctly, there's some code in common.asp that would demonstrate how it's done.