Application servers are used (among other things) to ensure scalability of a web site. Although they typically offer a all-in-one solution (with embedded servlet-enabled web engine, embedded JNDI registry and other services), at deployment they should be used on different tiers: one machine for the web tier, another for the application server, with often a third one for the database, while the registry might be elsewhere.
Configuring systems (like JBoss or Glassfish) to use an external Tomcat instead of the one they embed is well documented on the net, as long as both the app server and the web server reside on the same computer. However, as soon as you try to move Tomcat on a different machine, you run into trouble. I gave students of my course the challenge of solving the problem. The first who was able to solve it, Massimiliano Battan, documented his solution on a blog (yes, I suggested students to start blogging about their solution. Some of them just posted problems and questions, and guess what? nobody responded. Clearly they misunderstood the role of a blog… Others did a good job blogging about the solution they found to their problems, contributing – thanks to search engines – to the global knowledge.)
Here is Massimilano’s solution: