A small program that runs as a BIOS extension, can be loaded from a floppy, or you can burn an eprom chip and insert it on your network card for a totally diskless client. It handles the DHCPD query and TFTP loading and then transfers control to the loaded image. It uses TCP/IP protocols.
There are two free implementations of TCP/IP net loaders: Etherboot and Netboot: Etherboot uses built-in drivers while Netboot uses Packet drivers.
cat floppyload.bin ne2000.lzrom > /dev/fd0