There are additional limitations to phase III nodes in a phase IV network. Phase III nodes do not have any idea of areas, and all node numbers must be below 256. (In short, phase III nodes use a single byte for node addresses.) So, a phase III node can only talk with other nodes within the same area, and only if those nodes have low node numbers. Another key point: Phase III doesn't support Ethernet -- it only handles point to point (DDCMP and equivalent) links. The network layer messages are different, too. For one thing, the init message has the older routing layer version number in it. For another, in the case of routers, the route vector is sent as a simple full vector rather than as a list of segments with new data (omitting unchanged data) as is done for Phase IV. It's certainly possible in principle for a Phase V node to understand Phase III routing layer messages; in practice, I doubt this was done and the rules certainly didn't require it. paul