NBD (Network Block Device) is a userland implementation that enables the creation and access of block devices over a network connection. The protocol allows a remote server to expose block devices that can be mounted and accessed as local block devices on client systems. The implementation includes both server and client components that facilitate network-based block device operations. The server component can expose local storage resources as network-accessible block devices, while the client component can connect to and utilize these remote block devices as if they were locally attached storage. The NBD protocol implementation supports various use cases in distributed computing environments, including cloud infrastructure, virtualization platforms, and distributed storage systems. The protocol operates at the block level, providing low-level access to storage resources across network connections. Multiple projects implement the NBD protocol, including nbdkit, libnbd, qemu, and lwNBD, each offering different features and capabilities for network block device operations. The protocol enables flexible storage architectures where storage resources can be centralized and accessed by multiple clients over network connections.