The installer may be booted using boot files placed on an existing hard drive partition, either launched from another operating system or by invoking a boot loader directly from the BIOS. On modern UEFI systems, the kernel may be booted directly from the UEFI partition without the need of a boot loader.
A full, „pure network“ installation can be achieved using this technique. This avoids all hassles of removable media, like finding and burning CD/DVD images.