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DECnet is a network protocol family developed by the Digital Equipment Corporation (DEC). It was originally developed to connect two PDP-11 microcomputers, but it eventually transformed into one of the first peer-to-peer network architectures in the 1980s. It was then built into VMS, DEC’s flagship operating system.
DECnet is a set of networking hardware and software products that make use of the DIGITAL Networking Architecture (DNA), a collection of documents that state the specifications of each layer of the architecture and describe the protocols operating in those layers.
Phase I of DECnet was released in 1974 and it only supported the PDP-11s running the RSX-11 OS, and the only communication method available was point-to-point.
In 1975, Phase II was released with support for 32 nodes that had differing implementations from each other, including TOPS-10, TOPS-20 and RSTS. It had a Fila Access Listener for file transfers, Data Access Protocol for accessing remote files and features for network management, but the communication between processors was still limited to point-to-point links.
Phase III was released in 1980 and this time support was increased to 255 nodes, now featuring both point-to-point and multi-drop links. An adaptive routing capability was introduced and it was now able to communicate with other network types such as IBM’s SNA through gateways.
Phase IV and IV+ were released in 1982 with support for a maximum of 64,449 nodes and includes Ethernet local area network support as the main choice for datalink. It included hierarchical routing, VMScluster support and host services. It also made use of an 8-layer architecture similar to the 7-layer OSI model, especially in the first lower levels. This made DECnet OSI compliant, but since OSI standards were not yet fully standardized at that period, Phase IV’s implementation was considered proprietary.