Tim,
Actually I just found a KB that discusses this very topic (see link below) - all I did was go to the NI knowledgebase and typed in "ethernet interfaces rt" into the search window and the first selection was the one I wanted.
http://pong.ni.com/public.nsf/websearch/A4D35697C9DFC879862569C40061D8BE?OpenDocument
Zenerell