What you are looking to do is actually impossible to achieve without custom javascript code. Below, you will find a simple example of how it can be done. Every link you have will need to call the javascript function, passing a url to it. Please, let me know if you have additional questions.
<html>
<head>
<script type="text/javascript">
function DoIframe(url_)
{
var obj = document.getElementsByTagName("IFRAME")[0];
obj.src=url_;
return false;
}
</script>
</head>
<body>
<a href="#" onclick="DoIframe('http://www.yahoo.com'); return false;">Try it out</a><br /><br />
<a href="#" onclick="DoIframe('http://www.google.com'); return false;">Try it out</a><br /><br />
<a href="#" onclick="DoIframe('http://www.dotnetnuke.com'); return false;">Try it out</a><br /><br />
<a href="#" onclick="DoIframe('http://marketplace.dotnetnuke.com'); return false;">Try it out</a><br /><br />
<a href="#" onclick="DoIframe('http://www.snowcovered.com'); return false;">Try it out</a><br /><br />
<IFRAME width="600" height="600" />
</body>
</html>