Booting from the network requires that you have a network connection and a TFTP network boot server (and probably also a DHCP, RARP, or BOOTP server for automatic network configuration).
The server-side setup to support network booting is described in Τμήμα 4.3, «Προετοιμασία Αρχείων για δικτυακή εκκίνηση με TFTP».