DECnet

Digital Equipment Corporation
Industry	Computer networking, computer hardware, software
Founded	November 7, 1957; 69 years ago in Massachusetts, United States
Founder	Ken Olsen, Harlan Anderson
Defunct	December 31, 1998
Fate	Acquired by Compaq (later HP)
Headquarters	Maynard, Massachusetts, United States
Products	DECnet, PDP series computers, VAX computers, OpenVMS operating system

DECnet represents a collection of network protocols created by Digital Equipment Corporation (DEC). Introduced in 1975 to link two PDP-11 minicomputer systems, it emerged as one of the earliest peer-to-peer networking models, positioning DEC as a dominant force in networking throughout the 1980s. Originally developed with three layers, DECnet was expanded in 1982 to align with the seven-layer OSI model.

From the outset, DECnet was embedded within DEC's principal operating system, OpenVMS. Subsequently, DEC broadened DECnet compatibility to include Ultrix, OSF/1 (which later became Tru64), and also extended support to Apple Macintosh and IBM PC systems running DOS, OS/2, and Microsoft Windows via the PATHWORKS software suite. This integration allowed these platforms to operate as terminal nodes within DECnet networks dominated by VAX computers.

While DEC was responsible for designing the DECnet protocols, versions from Phase II onward were released as open standards. This openness facilitated implementations beyond DEC’s ecosystem, such as for FreeBSD and Linux. Nevertheless, Linux kernel support for DECnet was abandoned in 2010 and ultimately removed in 2022.

Evolution

DECnet consists of both hardware and software components that implement the DIGITAL Network Architecture (DNA). This architecture is thoroughly documented, detailing each layer and its associated protocols. Although network analyzers often categorize all DIGITAL protocols under "DECnet," certain non-routed DIGITAL protocols—such as LAT, SCS, AMDS, and LAST/LAD—do not belong to either DECnet or DNA.

The progression of DECnet reflects the development of DNA, which started in the early 1970s alongside IBM's Systems Network Architecture (SNA). DNA has undergone several stages:

1970–1980

Phase I (1974) Enabled communication between two PDP-11 machines running RSX-11, or among a few PDP-8 systems using RTS-8, connected via point-to-point (DDCMP) links.

Phase II (1975) Extended networking capabilities to support up to 32 nodes with interoperable versions on RSTS, TOPS-10, TOPS-20, and VAX/VMS. Communication remained point-to-point. Introduced features such as downline loading through MOP, file transfer using File Access Listener (FAL), remote file access via Data Access Protocol (DAP), task-to-task communication interfaces, and network management tools.

Phase III (1980) Expanded network size to accommodate up to 255 nodes with 8-bit addressing over both point-to-point and multi-drop links. Added adaptive routing mechanisms, record access functions, a network management framework, and gateways connecting to other networks including IBM’s SNA and the CCITT Recommendation X.25.

**DECnet Phase IV protocol suite**
Application	DAP: Data Access Protocol CTERM: Command Terminal
Network Management	NICE: Network Information (and) Control Exchange MOP: Maintenance Operations Protocol
Session	SCP: Session Control Protocol
Transport	NSP: Network Service Protocol
Network	DRP: DECnet Routing Protocol
Data link	DDCMP: Digital Data Communications Message Protocol Ethernet, Token Ring, HDLC, FDDI, ...
Physical	Ethernet, Token Ring, FDDI, ...

1981–1986

Phase IV and Phase IV+ (1982) were initially developed for RSX-11 and VMS, subsequently extending support to TOPS-20, TOPS-10, ULTRIX, VAXELN, and RSTS/E. These versions accommodated networks with up to 64,449 nodes using 16-bit addressing, incorporated advanced datalink protocols surpassing DDCMP by adding Ethernet LAN support, hierarchical adaptive routing, VMScluster integration, and host utilities such as CTERM enabling remote logins akin to Telnet. Digital also introduced the PATHWORKS client (PATHWORKS 32), which implemented DECnet Phase IV functionality on DOS and Microsoft Windows systems through Windows Server 2003.

Phase IV employed an eight-layer model similar to the OSI reference framework; however, many protocols remained proprietary since OSI standards were still under development. Its Ethernet implementation was distinctive in modifying physical addresses to the form AA-00-04-00-xx-yy, directly mapping DECnet network addresses onto LAN hardware addresses. This design eliminated the need for ARP but prevented multiple network interface cards from a single DECnet node from operating on the same LAN segment.

While originally designed for VAX/VMS and RSX-11 platforms, DECnet expanded to nearly all DIGITAL operating systems except RT-11. Additionally, DECnet protocol stacks became available on Linux and SunOS systems and received support from Cisco and other vendors. Complete Phase IV documentation remains publicly accessible.

Concurrently, Digital developed the proprietary LAT protocol to facilitate serial terminal connections via terminal servers. LAT shared physical and datalink layers with DECnet and utilized MOP for downloading server images.

Further improvements culminated in DECnet Phase IV+, which maintained full interoperability with Phase IV implementations.

1987 & beyond

Phase V and Phase V+ (1987) introduced support for networks without architectural constraints, featuring a novel management paradigm, distributed naming services, and enhanced performance. This phase marked a shift from proprietary networking towards an Open Systems Interconnection (OSI) framework by adopting ISO standards to enable multi-vendor interoperability while preserving backward compatibility with DNA Phase IV through a hybrid architecture combining DNA and OSI protocol "towers" atop a unified transport layer. Transparent transport links to TCP/IP were incorporated using IETF RFC 1006 (OSI over IP) and RFC 1859 (NSP over IP).

The product was renamed DECnet/OSI to emphasize its OSI compatibility, later evolving into DECnet-Plus as TCP/IP protocols were integrated.

DEC Easynet

Digital Equipment Corporation's internal corporate network, Easynet, originated from the Engineering Net (E-NET). It grew substantially from more than 2,000 nodes in 1984 to 15,000 nodes spanning 39 countries by 1987, ultimately reaching 54,000 nodes by 1990.

The DECnet Internet

DECnet linked numerous scientific research institutions into an international mesh known as the DECnet Internet. This network encompassed entities such as the U.S. Space Physics Analysis Network (US-SPAN), European Space Physics Analysis Network (E-SPAN), and the Energy Sciences Network. By 1989, it comprised over 17,000 nodes. Inter-network routing relied on "poor man's routing" or address translation gateways. In December 1988, VAX/VMS systems on the DECnet Internet were affected by the Father Christmas worm.

CCNET

CCNET, standing for Computer Center Network, was a DECnet-based network that connected university campuses in the eastern United States during the 1980s. It enabled the sharing of system software among various DEC computer systems. By March 1983, the network linked Columbia University, Carnegie Mellon University, and Case Western Reserve University. By May 1986, it had expanded to include New York University, Stevens Institute of Technology, Vassar College, and Oberlin College, with further institutions joining in subsequent years.

Hobbyist DECnet networks

DECnet hobbyist networks have continued to operate into the 21st century, notable examples being:

HECnet
Italian Retro DECnet