>But I want to attach a BSD machine to a network. BSD only supports
>ethernet, so A/UX won't help me. The machine that is already on the net

What??? A/UX has the complete Berkeley networking suite in its kernel.
MacOS *does not*. A/UX contains several ethernet device drivers. The
machine you mentioned was a Centris/Quadrs 650 a perfect vehicle for
A/UX.

>won't take BSD, but it has to keep macOS. can't bring down both my primary 

A/UX runs MacOS within a single monolithic Unix process. But, *sigh* support
ends at System 7.0.1+ but it still works great as a server for other Macs.

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packets from a ppp connection only it's attached network.  The IP would
have to be wrapped up in appletalk and then just sent on it's way, as I see
it. Is there some greater complication? It's a 2-network, 2-card situation.
The hardware is there, but the OS won't handle both at once.                    
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Well if you want to use PPP you would probably be better off with a separate
high speed serial card. As to Unix supporting routing between PPP and ethernet
based TCP/IP. A/UX would do it as well if not better than most if only
*sigh* we build a PPP kernel from Kirk Smith's dp package. So if your
point is that you need a separate box to route PPP conectivity across
ethernet the Apple Internet Router is the *wrong* tool for this job. You
need a single (serial) port gateway/router/terminal server box. I've seen
a few companies offering this in Mac catalogs and in fact MacUSER recently
tested several of them. I would select one of those. MacOS with MacTCP
running the the Internet router software is not the appropriate tool in
this case.

-- Peter Brewer