All I can say is check out the Network Preference Pane and go to Network Port Configurations.
[robg adds: And I'll add a bit more ... click on New, and you can then select "Built-in FireWire" and add it to use IP over FireWire. THere's also a "6 to 4" item, but I have no idea what that's for.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031027011901587