Each Raspberry model makes it differently.
There's possibility to update firmware for Raspberry 4. Factory releases Raspberry 4 with network boot turned off.
To turn it on you should update firmware. Download from: wtware.com/files/raspberry/
pi4netboot-diagnostic-eeprom-nnn.zip archive for the latest version. Unpack all the files from archive
into clean SD card's root. Insert card intoRaspberry 4 and turn it on. If succeed, in several seconds green light will blink frequently.
Turn Raspberry Pi off and eject SD. Turn raspberry off and eject SD. Updated in such way Raspberry 4 will boot only by network.
When you turn it on on the screen you'll see information about computer and boot progress. If you just see black screen,
something is wrong with display or display cable. Initial image appears instead of network state.
There's another firmware for network boot, without diagnostic information. On the screen you'll see nothing during boot.
It's useful when everything works and users are frightnened by too many words. But id network boot fails,
it will be more complicated to find out what is wrong. Download in the same place: wtware.com/files/raspberry/
pi4netboot-eeprom-nnn.zip archive without word diagnostic.
To bring back SD boot, download pi4default-eeprom-nnn.zip archive. Another way to return SD boot is
to run Raspberry Pi Imager
from official Raspberry site and write to SD
Bootloader EEPROM configuration > Bootloader > SD card boot.
We don't recommend to boot Raspberry 4 by network, if you use local Google Chromium, especially on Raspberry 1GB RAM. Chromium is large. When you use network boot, all Chromium files are stored in memory, so remains not enough memory for work. When Pi boots from local SD, WTware unpacks many files to SD, so remains more free memory.
WTware DHCP autodetermines data, that Raspberry needs for network boot. If you use another DHCP, specify in 066 DHCP option TFTP server IP address.
Raspberry ignores 067 DHCP option and always boots from TFTP root or from corresponding to Raspberry serial number directory. WTware TFTP knows about that and sends Raspberry WTware version, that is specified in WTware Configurator. If you use another TFTP, find in your TFTP logs, where Raspberry searches for boot files. Use WTware zip archive, keep it's structure of directories.
If you need to make changes to config.txt or cmdline.txt
for one Raspberry, that boots from WTware TFTP, copy it from directory
(instead of 6.0.4 specify your WTware version):
C:\Program Files (x86)\WTware\TFTPDROOT\6.0.4\pi2\localboot\into:
C:\ProgramData\WTware\Terminals\{MAC-address of your Pi}\
And in terminal directory make changes to files, needed only for this terminal. Firstly WTware TFTP
searches for config.txt or cmdline.txt files in terminal directory
and if fails, sends common files.
We don't recommend to boot Raspberry 2b, 3b and 3b+ by network, when your network is more complicated than one switch, connecting server and several Raspberries. Raspberry bootloaders up to Pi 4 version contain errors. Besides problems, that result in "Raspberry failed to boot", sometimes while boot by network Raspberry 2b, 3b and 3b+ use IP owned by another computer, not the determined for Raspberry by DHCP IP, which leads to failure of other computer with this IP.
Raspberry 3b+ boots by network only without SD. Or fails to boot, then this Pi3b+ has no chance to boot by network.
We don't recommend to boot Raspberry 2b, 3b and 3b+ by network. Embedded into processor code sometimes allows to boot without SD. By default this code is turned off. To turn it on download pi3netprog.zip. Unpack it to clean SD. Boot Raspberry 3b, wait for the message, that configuration succeed. Then it should boot by network without SD. At boot start will be delay for about 10 seconds.
If won't boot, you may try to use SD, containing only bootcode.bin file, the same as for Raspberry 2b.
We don't recommend to boot Raspberry 2b, 3b and 3b+ by network. Raspberry Pi 2b is not able to boot without SD. It needs SD card, containing only bootcode.bin file. This file you may copy from boot SD, it's the same for all Raspberry models.