Who told you that you have to use the URL with "NotFound"?! Use
http://www.asp.net/Any_gibberish_at_all[
^], and you will get the same result. This is a very usual setting for a HTTP server and the site. This error 404 means "not found". When you host the site, you can define the
file names for all usual HTTP error cases and put some real HTML files under these names.
Please see:
http://en.wikipedia.org/wiki/List_of_HTTP_status_codes[
^],
http://en.wikipedia.org/wiki/ERROR_404[
^].
Now, if you do Web development yourself, this is your obligation to be aware of that, configure those file names (or ask you Web hosting customer service), develop appropriate error-response HTML files and put them under these names. Failure to do so is considered as unprofessional and sloppy. Just do it.
[EDIT]
See also my comment to the comment to the question by Trak4Net: on your error pages, you can provide a link to a top page of the site, site map, or both, but hardly anything else.
—SA