Network block device
In Linux, a network block device is a device node whose content is provided by a remote machine. Typically, network block devices are used to access a storage device that does not physically reside in the local machine but on a remote one. As an example, the local machine can access a fixed disk that is attached to another computer.
Kernel client/userspace server 
Technically, a network block device is realized by two components. In the client machine, where the device node is to work, a kernel module named
nbd controls the device. Whenever a program tries to access the device, this kernel module forwards the request to the server machine, where the data physically resides.
On the server machine, requests from the client are handled by a userspace program called
nbd-server. This program is not implemented as a kernel module because all it has to do is to serve network requests, which in turn just requires regular access to the server filesystem.
If the file
/tmp/xxx on ComputerA has to be made accessible on ComputerB, one performs the following steps:
nbd-server 2000 /tmp/xxx
modprobe nbd nbd-client ComputerA 2000 /dev/nbd0
The file is now accessible on ComputerB as device
/dev/nbd0. If the original file was for example a disk image, it could be mounted for example via
mount /dev/nbd0 /mnt/whatever.
modprobe nbd is not necessary if module loading is done automatically. Once the module is in the kernel,
nbd-client is used to send commands to it, such as associating a given remote file to a given local nb device. To finish using
/dev/nbd0, that is, to destroy its association with the file on other computer, one can run
nbd-client -d /dev/nbd0 on ComputerB.
In this example, 2000 is the number of the server port through which the file is made accessible. Any available port could be used.
See also 
- ISCSI: The "target-utils" iscsi package on many GNU/Linux distributions the tgtd can configure the backing storage of a LUN to be any block device (disk, partition, etc). This has widest adoption amongst IP-based block device presentation protocols.
- Loop device: a similar mechanism, but uses a local file instead of a remote one
- DRBD: Distributed Replicated Block Device is a distributed storage system for the Linux platform
- NBD Mailing List
- Network Block Device on Sourceforge
- The Network Block Device, the Linux Journal
- ggate in FreeBSD handbook, FreeBSD GEOM Gate presentation
- Enhanced Network Block Device
- nbd-verify aims to be a test kit to verify the protocol implementation as well as a data handling tester of NBD servers
- Miroslav Kripac and Masaryk University Brno (December 10, 2002). "Implementing Oracle Real Application Clusters Using Network Block Device Technology". Retrieved May 22, 2013.
- "iSCSI Adoption Continues its Upward Path".