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".
The documentation for these products also discusses how to set up the client
software using MacTCP or Open Transport.