The netinst CD image is a popular image which can be used to install bullseye with the debian-installer. This installation method is intended to boot from the image and install additional packages over a network; hence the name “netinst”. The image has the software components needed to run the installer and the base packages to provide a minimal bullseye system. If you'd rather, you can get a full size CD/DVD image which will not need the network to install. You only need the first image of such set.

Download whichever type you prefer and burn it to an optical disc. To boot the disc, you may need to change your BIOS/UEFI configuration, as explained in Section 3.6.1, “Invoking the BIOS/UEFI Set-Up Menu”.

It's also possible to install from removable USB storage devices. For example a USB keychain can make a handy Debian install medium that you can take with you anywhere.

The easiest way to prepare your USB memory stick is to download any Debian CD or DVD image that will fit on it, and write the image directly to the memory stick. Of course this will destroy anything already on the stick. This works because Debian CD/DVD images are “isohybrid” images that can boot both from optical and USB drives.

There are other, more flexible ways to set up a memory stick to use the debian-installer, and it's possible to get it to work with smaller memory sticks. For details, see Section 4.3, “Preparing Files for USB Memory Stick Booting”.

While booting from USB storage is quite common on UEFI systems, this is somewhat different in the older BIOS world. Some BIOSes can boot USB storage directly, and some cannot. You may need to configure your BIOS/UEFI to enable “USB legacy support” or “Legacy support”. The boot device selection menu should show “removable drive” or “USB-HDD” to get it to boot from the USB device. For helpful hints and details, see Section 5.1.1, “Booting from USB Memory Stick”.

It's also possible to boot debian-installer completely from the net. The various methods to netboot depend on your architecture and netboot setup. The files in netboot/ can be used to netboot debian-installer.

The easiest thing to set up is probably PXE netbooting. Untar the file netboot/pxeboot.tar.gz into /srv/tftp or wherever is appropriate for your tftp server. Set up your DHCP server to pass filename pxelinux.0 to clients, and with luck everything will just work. For detailed instructions, see Section 4.5, “Preparing Files for TFTP Net Booting”.

It's possible to boot the installer using no removable media, but just an existing hard disk, which can have a different OS on it. Download hd-media/initrd.gz, hd-media/vmlinuz, and a Debian CD/DVD image to the top-level directory of the hard disk. Make sure that the image has a filename ending in .iso. Now it's just a matter of booting linux with the initrd. Section 5.1.5, “Booting from Linux using GRUB” explains one way to do it.