Firewall protects, so what directs?

[Thomas Cook <sysadmin@black-gear.com>]

I have spent the last few months constructing an ipchains firewall for my
computer lab.  I finally got everything working a week or so ago, but I
realized there is noting telling things where to go.

My firewall divides my network into an internal lab (10.0.0.0/24, all
ip_forward and MASQ on the firewall), and a DMZ for my servers
(10.10.0.0/24).  The firewall tells all the packets where they can and cant
go, but how do I tell packets where they should go?  For example...

Lets say my external ip is 1.2.3.4.  So someone on the internet plugs
1.2.3.4 in their browser.  The browser contacts my firewall's external
interface asking for connect on port 80.  How do I tell my firewall to
direct that www request to 1.2.3.4 into a request to 10.10.0.10 port 80 (my
apache server)?

-Tom