Ooh Ooh i think i get it ... You'll have to use some server side scripting.
$page = ($_GET["page"]==''?'subsection1.htm':$_GET["page"]) . '.htm';
<iframe src="<?=$page?>"> You cannot view inline frames </iframe>
The php creates a dynamic page reference so you can put anything you want by assigning it into the variable page and passing it in the url. This is assuming you have php installed and working.
<a href="page1.php?page=subsection1">Page1: Subsection1</a>
<a href="page1.php?page=subsection2">Page1: Subsection2</a>