NAS is different in that the File I/O request is extended over the LAN to a centralized file system where the Block I/O conversion takes place. As all file requests are extended to this one file system, NAS provides a generic platform that provides not only networked storage access, but also provides data sharing across heterogeneous hosts.
NAS architectures can be delivered either in a standalone appliance where scalability is limited and no protection is provided against device failure, or as a gateway device that provides the benefits of NAS but with the scalability and protection options of a SAN
DAS and SAN are both Block I/O based architectures NAS is a File I/O based architecture