You'll need to set up a PXE boot server and then set one of the NICs on the server to support PXE boot (can be set in the bios of the server).
You'll want to Google for PXE boot.
As you're asking about how to do this it would seem you're completely unfamiliar with PXE booting. The time and effort you'll spend on setting it up is probably more costly than to just go out and buy an optical drive (either an internal one or a USB version).
There's always the option to use the virtual media capabilities of the DRAC if you have a DRAC in the server.
Member since 2003