A tough one! W2K vs NT 4.0

Here is the setup: A windows 2000 server that serves as a NAT (network address translation) with 2 NIC cards. One card has an internal ip address and the other a public ip address. The internal card is pluged one hub and the external card is plugged in a switch. A router that gives access to the internet is also pluged in the switch and about 20 computers are pluged in the internal hub and 10 computers pluged in the switch with public ip adresses.

Here is the problem: we have 5 servers that have public ip address in another building. 2 on nt 4.0 and 3 on W2K. On an internal computer1, we open windows explorer to see all the mapped drives. All mapped drives are connected (no red X on them). As soon as another internal computer2 searches and finds an external server(W2K), a red X appears on the mapped drive to this server(W2K) on the first computer1.

When we do the same exercise execpt we search for an external server on NT 4.0, no red X appears.

When we do the same execise with a computer that is pluged in the switch with an external address... no red X either!!

We went as far as making a dual boot on an external server NT 4.0 and W2K. When we boot in W2K the problem occurs, when we boot in NT 4.0.... no problems! It is the same server, with the same setup and the same IP adress.

Here is my question: Why do I get a red X on my mapped drives on internal computers? And why does it only happen when the server is on W2K?

Thank you