Need help with DB design/model

[Chris Bassett]

I have a piece of my database that I need some assistance modeling/designing:

It's a shipment tracking system where you have order numbers, and you can add one or more tracking numbers to go with the orders.

Here is how our tracking numbers work:
* Each carrier uses a tracking number system (so, no order is shipped without a tracking number)

* You can have one or more orders that are shipped on one single tracking number (this is the part I'm having a problem with)

* Once a tracking number is used, it cannot be re-used

I'm trying to think of how I can enforce this at the database level, using primary keys, unique constraints, etc.

Any ideas? Let me know if you need more info.