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Tru64 is a Unix variant produced originally by Digital Equipment Corporation for use on the Alpha platform which they also invented. It has several names. Version 1.0 through 3.0 used the moniker “OSF/1” but in the 4.0 versions it was called “Digital Unix” (ironically called that right after Compaq bought DEC). Then in version 5 and after it was called Tru64 to reflect it's 64-bit internals.
DECNet Setup for Tru64 and Digital Unix explains how to setup DECNet Phase V on Tru64.