Multihoming/Multi-ISP question

[Lance Westerhoff]

Hello-

We recently switched from one ISP to another, and as our domain and  
whatnot are changed over, I was hoping to listen to both IPs at  
once.  I have a Mac OS X Server (10.4) with three working ethernet  
cards: one for each of the internet connections, and one for an  
internal network.  The machine also acts as our DHCP/NAT box for our  
small office network.  Again, all of this works except one thing: I  
can't listen to both internet connections at once.  Basically, if I  
set one up in the Network Control Panel as the top connection, the  
other internet connection goes down.  I figured that one of the  
interfaces is down, but according to ifconfig they are both up but I  
haven't been able to use ifconfig to have accept packets from both.   
I must be missing something stupid.  I have spent hours looking  
around to no avail.  It doesn't really seem to be an IPalias  
situation since technically it isn't an alias to another interface.   
When I try to set up an alias using ifconfig, no matter what I do (to  
either interface) seems to do the trick.

Unfortunately, this is just a temporary need until everything gets  
switched over/tested.  I've set up a similar situation without  
trouble on Linux before so I can't imagine OS X Server can't do the  
same thing!

Thank you for your time and insights!

-Lance