Creating Duplicate domains

Okay, let's see if I can explain this properly. My company is upgrading our
NT boxes. So what I did was make an image of our primary and backup domain
controllers. Then I used identical hardware and set up a closed-off network
to mimic the company network.

Theoretically, shouldn't a computer hooked up to this mimic network log in
just as if it were on the real network? I used a laptop that was part of the
real domain and network and then plugged it into the switch (with closed
network) and it keeps telling me that the domain isn't available.

I have DNS, DHCP, and WINS set up for the network. And any new computers i
configure to log on to the mimic domain log on just fine. However, computers
that were set up for the real domain just can't find the domain.

Why is this?