Routing issue

[Web Walrus (Robert Wall)]

> > > > ifconfig_dc0 inet 1.2.3.4 netmask 255.255.255.248
> > > > ifconfig_dc0_alias0 inet 2.3.4.5 netmask 255.255.255.248
> > > > defaultrouter="1.2.3.1"
> >
> > It's not on the same network; that's the problem.  Two complete separate
> > networks, same interface card.  The issue is that one of the networks
> > works, and the other doesn't, depending on what network the default router
> > happens to be on.
>
> In general, you're going to need a mechanism for dynamically routing
> packets in order to make this sort of setup work.  For most setups,
> you'ld need the co-operation of your ISP to make things work as well.

The situation is this - there are 4 servers that are on one network.  I'm
trying to switch them over to another network, but I need to do it without
downtime.  Therefore, I need to have both IPs completely active and
functional simultaneously.

Would the situation be any easier if I put one of the networks on a
separate NIC?

Is there any way to determine what IP/interface a connection came in on,
and continue to use that IP/interface for the outbound packets?  Maybe
with static routes or something of that nature?

The thing is, I used this exact setup (albeit on two different network
cards) on a FreeBSD 2.x box quite a ways back, for the same purpose
(switching networks), and it was working fine.