Port forwarding is setup in your gateway/NAT.
Most of these devices have some kind of web interface for configuring them.
The concept of port forwarding is the gateway will route requests to its 'public' address on a specific port to a specific internal address on the same port.
FYI: SQL server 2000 using port 1433
Setup your gateway to forward port 1433 to the internal static address you setup for your SQL server, and direct your VB6 application to connect to your gateway/nats 'public' address and you should be in business.
There could also be something else going on here if your being disconnected internally and externally. If your using a named instance of SQL server, your port might be configured dynamically. You'll need to set this to a static port in order to use port forwarding.