Before TCP/IP, DEC had DECnet. It start off as peer to peer networking and now it can network thousands of machines. There is a hobby DECnet installation called HECnet. DECnet runs on the VAX under both VMS and ULTRIX, the PDP-10 under TOPS-20, and the PDP-11 under RSX-11M+ and RSTS/E. There is also evidence that DECnet existed for the Rainbow 100, The PDP-8, and RT-11 on the PDP-11. Also, there is DECnet support for the older DOS and Windows PC under the name Pathworks. Also, DECnet support used to be available for Linux but all remnants of it have been removed from recent kernels. From what I have heard though, there is at least one project underway to restore it. Also, for emulation purposes, there is a Python-based DECnet router called PyDECnet https://github.com/pkoning2/pydecnet.