Probably depends on your distro, but you could try
/etc/rc.d/init.d/network restart
If it's not in there, it (or something similar) should be pretty close.
If you ever want to restart just about everything without rebooting the machine, try 'telinit 1', then 'telinit 5' once it's done with that.
Takes you down to single-user mode and back up, which automatically kills off and restarts most of the services.