It is not because out of laziness or convenience a great many number of URLs bear a close resemblance to the file which is finally served by the web-server, that there is any necessary link between the URL and such file. As a matter of fact, your above CGI-script is most likely not called 'undersite/1.txt' or '1.txt', but rather 'something.pl' or 'something.cgi' or anything else you have your web-server configured to accept as the link to your script.

It has been suggested that perhaps you should use the file:// URL-scheme, but beware, this is likely to work only if the file is on the same computer the web-page is being viewed upon. RFC 1738 says as follows about this scheme:

3.10 FILES

The file URL scheme is used to designate files accessible on a particular host computer. This scheme, unlike most other URL schemes, does not designate a resource that is universally accessible over the Internet.

A file URL takes the form:

    file://<host>/<path>

where <host> is the fully qualified domain name of the system on which the <path> is accessible, and <path> is a hierarchical directory path of the form <directory>/<directory>/.../<name>.

For example, a VMS file

  DISK$USER:[MY.NOTES]NOTE123456.TXT

might become

  <URL:file://vms.host.edu/disk$user/my/notes/note12345.txt>

As a special case, <host> can be the string "localhost" or the empty string; this is interpreted as `the machine from which the URL is being interpreted'.

The file URL scheme is unusual in that it does not specify an Internet protocol or access method for such files; as such, its utility in network protocols between hosts is limited.

The very last sentence is of course the most important.