There are at least two products that will perform
TCP/IP
"encapsulation"
into Appletalk using a Mac:
1. A current product called IPNetRouter. See:
http://www.sustworks.com/site/ipr_guide/localtalk.html
It will run on all current Macs.
2. An old Apple product called "Apple IP Gateway". See:
http://til.info.apple.com/techinfo.nsf/artnum/n16705
This product will not run on PCI Macs or on Macs running
"Open Transport".
Another product is Vicom Internet Gateway... it's been
split into two products now, one carrying the same name
includes the ability to block access to specific web sites
and has CyberNot filter capability.
A more stripped-down version is called SoftRouter. Both
products allow you to multi-home the Mac, and it then
routes Localtalk to TCP/IP and vice-versa.
Nice product; we use it on a Mac to connect our entire
office to the Internet over a measly V.90 link.
-dq