Re: Lies, damned lies, and statistics that don't lie.
"IIRC DECNet relied on - or assumed - that the MAC addresses were the subset allocated to DEC. Trying to get HP-UX boxes talking to a VAX with a VAX-oriented management we had to buy a DECNet package for HP-UX. When it was installed it promptly changed the MAC (which was programmable) to look like DEC. That confused all the clients until their caches caught up."
I think you'll find DECnet used locally-assigned MAC addresses rather than addresses "allocated" to DEC. So that's why the MAC changed on you once you fired up DECnet. The locally-assigned MACs encoded the DECnet station ID I think - so you didn't need an ARP equivalent.