Whatever web server you're using (IIS, Apache, etc.) needs to have "index.html" and "index.htm" listed as one of the "default" documents. That way, if a URL points to a folder without a doc name, the web server knows to look for a default document and load that. I don't know about Apache, but in IIS you can set the order in which default documents are attempted (ie, index.html first, index.htm second, home.asp third, etc.)
^_^