URL masking mean that you would like to hide the actual path of the website file. In order to do that, you may always use frame tag.
For example, you would like your main page to be linked to other website or subfolder, then place the below script in a file and name the file as index.html in your website root directory.
<HTML>
<HEAD>
<TITLE>This is test </TITLE>
</HEAD>
<FRAMESET>
<FRAMESET rows>
<FRAME src=”http://www.domain.com/file.ext”>
</FRAMESET>
</HTML>