/etc/modprobe.conf file. However, it is sometimes necessary to explicitly force the loading of a module at boot time.
/etc/rc.modules file at boot time, which contains various commands to load modules. The rc.modules should be used, and not rc.local because rc.modules is executed earlier in the boot process.
foo module at boot time (as root):
# echo modprobe foo >> /etc/rc.modules # chmod +x /etc/rc.modules