I'm looking to do precisely the same thing. Unfortunately the 'route' options are explained as tersely as all others in the man pages, which is unfortunate.
I can't identify a meaningful way to accomplish this via the Network preference pane, and I am at a loss to grasp the intricacies of a routing table at-a-glance.
If there's anyone who's actually managed to accomplish this, particularly in 10.6.4, it'd be great to have some pointers. Thanks!
UPDATE: A discussion on this thread:
seems to have addressed part of the issue for me. Specifically the OP mentioned using 'sudo route add' to modify the routing table. In his case the command was:
sudo route add 10.0.0.0/8 <intranet gateway>
That works for my setup, since our intranet IP addresses are all 10.x numbers. (Note that I believe you have to have your root user active to make this work.) Now, anything I access directly by IP address is handled via my wireless connection to the intranet, while everything else seems to be passing through the internet.
In the above example, BTW, the
<intranet gateway> would be the IP address of the router for the 10.* connections, as given in the Network preferences pane. Hence, if your router IP was 10.32.0.1, you'd do this in a terminal window:
sudo route add 10.0.0.0/8 10.32.0.1
I hope this is of some use to you, as it was for me - though it doesn't handle internal domain resolution ('nw.company.com', for instance), as far as I can tell.