Imitating a virtual dir with a querystring parameter

[Alan]

Can someone give me an idea of how the technique that lets a querystring
parameter look like a virtual directory reference works.

You can see this in action at sites like [url]www.aspfaq.com[/url], where
[url]http://www.aspfaq.com/2161[/url] is actually a reference to
[url]http://www.aspfaq.com/show.asp?id=2161[/url]. Is it just a case of using a custom
404 processor to pick up the trailing identifier and causing a redirect to
show.asp, or is something else involved?

Cheers,

Alan