Older PA-RISC machines use rboot for booting over the network. Newer machines may also support bootp/dhcp/tftp, but I haven't had access to today's PA-RISCs...

Additionally to that, PA-RISCs also boot off CD-ROM drives (and eventually even from SCSI tapes). This is particularly useful if your PA-RISC only got exotic network interfaces that you can't easily serve connectivity for (like FDDI).