I'm going to explain what happens in my system; I hope it helps you (it could be not exactly the same in your case)
When my computer begins the "default runlevel" (which is "4", for X11) it calls to "/etc/rc.d/rc.M"; there are a couple of lines telling:
# Initialize the networking hardware. If your network driver is a module
# and you haven't loaded it manually, this will be deferred until after
# the hotplug system loads the module below.
if [ -x /etc/rc.d/rc.inet1 ]; then
so, if the file "/etc/rc.d/rc.inet1" exists and is executable, it runs. And this is the script which starts and stops the network.