Editing DECnet (section)

===1981–1986===
'''Phase IV'''<ref>{{cite web
   |url=https://www.cisco.com/c/en/us/td/docs/ios/decnet/configuration/guide/2_xe/dec_2_xe_book/overview_decnet.pdf
   |title=Overview of DECnet}}</ref><ref>{{cite web
       |url=http://linux-decnet.sourceforge.net/docs/doc_index.html |title=The DECnet Phase IV Specifications}}</ref> and '''Phase IV+''' (1982).
Phase IV was released initially to [[RSX-11]] and [[OpenVMS|VMS]] systems, later [[TOPS-20]], [[TOPS-10]], [[ULTRIX]], [[VAXELN]], and [[RSTS/E]] gained support. Support for networks of up to 64,449 nodes (63 areas of 1023 nodes) with 16-bit addresses, datalink capabilities expanded beyond DDCMP to include [[Ethernet]] [[local area network]] support as the datalink of choice, expanded adaptive routing capability to include hierarchical routing (areas, level 1 and level 2 routers), [[VMScluster]] support (cluster alias) and host services (CTERM). CTERM allowed a user on one computer to log into another computer remotely, performing the same function that [[Telnet]] does in the [[TCP/IP]] protocol stack. Digital also released a product called the PATHWORKS client, and more commonly known as the PATHWORKS 32 client, that implemented much of DECnet Phase IV for DOS, and 16 and 32 bit Microsoft Windows platforms (all the way through to Windows Server 2003).

Phase IV implemented an 8 layer architecture similar to the [[OSI model|OSI]] (7 layer) model especially at the lower levels.  Since the OSI standards were not yet fully developed at the time, many of the Phase IV protocols remained proprietary.

The Ethernet implementation was unusual in that the software changed the physical address of the Ethernet interface on the network to AA-00-04-00-xx-yy where xx-yy reflected the DECnet [[network address]] of the host. This allowed ARP-less LAN operation because the LAN address could be deduced from the DECnet address.  This precluded connecting two [[Network interface controller|NICs]] from the same DECnet node onto the same LAN segment, however.

The initial implementations released were for VAX/VMS and RSX-11, later this expanded to virtually every [[operating system]] DIGITAL ever shipped with the notable exception of [[RT-11]].   DECnet stacks are found on Linux, SunOS and other platforms, and Cisco and other network vendors offer products that can cooperate with and operate within DECnet networks.  Full DECnet Phase IV specifications are available.

At the same time that DECnet Phase IV was released, the company also released a proprietary protocol called [[Local Area Transport|LAT]] for serial terminal access via [[Terminal server]]s. LAT shared the OSI physical and datalink layers with DECnet and LAT terminal servers used MOP for the server image download and related bootstrap processing.

Enhancements made to DECnet Phase IV eventually became known as DECnet Phase IV+, although systems running this protocol remained completely interoperable with DECnet Phase IV systems.