With NAS, file I/O is redirected over a network. File I/O means commands to open and close files and read and write bytes of data from files. Contrast this with local file storage or SAN, which is block-based I/O. With block-based I/O, the host must tell the storage controller exactly what logical blocks to read or write. With NAS, the host tells the NAS appliance what file to open. With NAS, the block-to-file abstraction layer occurs in the NAS appliance instead of in the host. A driver in the client system interprets each file I/O request to determine if it should be conveyed over the LAN. If so, it uses the underlying protocol stack and system functions to transmit the file request over the LAN to a NAS system, which performs the requested actions.