Looking for network gateway through Mac

[labtek@cs.yale.edu (Tom Griest)]

Our local schools have a bunch of Apple-IIgs machines connected via
a slow network which I believe is called "phonenet" using (I believe)
Appletalk protocols to a Mac which acts as a file/print server.

What I want to do is add an Ethernet adapter to the Mac and terminal
emulation software to the Apples, and allow them to telnet to
a host which can at least provide text-mode Internet services.
This will also probably require s/w on the Mac to relay the
packets onto Ethernet using TCP/IP.

Has anyone done this?  Or can you point me to a solution path?

-Tom