Which servers do web applications run on?

As far as I know, SharePoint runs IIS web sites for each web application on any server that has "SharePoint Foundation Web Application" running on it. And this is an "all or nothing" operation - you get all the web applications or none. 

I've always been uneasy about this because you need the Web Application on the Central Admin server, but then that puts *all* your web apps on the server, which is unnecessary. 

Is there a way to assign web apps to specific servers? 

Philo Janus, MCP Bridging business & Technology: http://www.saintchad.org/ Telecommuter? http://www.homeofficesurvival.com/ Author: Pro InfoPath 2007 & Pro InfoPath 2010 Pro PerformancePoint 2007 Pro SQL Server Analysis Services 2008 Building Integrated Business Intelligence Solutions