How unique is the IIS sessionid on IIS 5, IIS6 and IIS7?

Can the Sessionid from a given IIS server repeat after a reboot? How likely is it that the same session id will be used a second time by two different clients on the same IIS server? On a different IIS server? Please see example:

SaveFile "Document开发者_StackOverflowName" & Session.SessionID & ".html"

Is there a difference between IIS 5.0, 6.0, and 7.0 regarding session id? thx

There is a higher than zero chance that SessionIDs will repeat, additionally there is no guarantee the same user will pick up the same SessionID on a repeat visit.

More information is available in this Microsoft KB article.