Routing Applications through two network connections

Question

I have a network wireless card which I use for internet and at the same time I use the airport to access the network printers in the LAN. When I try to make a remote desktop connection it goes through the wireless card and I need it to go through the airport.
I tried making airport the first one in the list, which solves the problem of connecting to the remote desktop, but then I cannot not access internet.
Is there any way one can route a certain IP address to go through an exact connection?
Thanks in advance for your help.
Maria

Answer 1

Network routing is based on destination IP address, not per-application.
Therefore it is not possible to tell Application A to use one connection while Application B uses another.
It is, however, possible to send all traffic for a specific IP address over one connection or another, but it won't matter which application tries to access that address, they'll all go the same way.
However, for the most part, this happens automatically. Since you only have one default gateway address on your system, the interface that gateway is associated with will be used for all non-local traffic, while each interface will handle the traffic for its own subnet.
What this means is that if your wired ethernet port is configured for 192.168.1.x/24 with a router address of 192.168.1.1 while your AirPort link is set for 10.1.1.x/24 then any traffic for anything in 10.1.1.x will go over the AirPort and anything else will go over the wired port.
The only reason to override this would be if you had a different router on the 10.1.1.x network that you want to use to get to specific addresses. For example, if you wanted to route to 123.45.67.89 via a wireless router at 10.1.1.1 you'd run the command:
route add 123.45.67.89 10.1.1.1
now all traffic destined for 123.45.67.89 will go over the wireless link to the router at 10.1.1.1 rather than the default router on en0.