Gentoo services are started by scripts in /etc/runlevels
The scripts reside in /etc/init.d
They are added or removed from the /etc/runlevels using the "rc-update" command.
If you refer back to the Handbook you will see numerous times where it tells you to add services using this command.
Most likely your Ethernet card is started by the "net.eth0" and it probably is in the /etc/runlevels/default directory ... if so then you can remove the startup service by the following command:
Code:
# rc-update del net.eth0