Storing partial credit card numbers

Possible Duplicates:

1. Best practices for taking and storing credit card information with PHP
2. Storing credit card details
3. Storing Credit Card Information

I need to store credit card numbers within an e-commerce site. I don't intend on storing the whole credit card number, as this would be highly risky. I would like to store at least the first five digits so I can later identify the financial institution that issued the card. Ideally, I would like to store as much of the credit number as I safely can, to aid any future cross-referencing etc.

How many digits, and which particular digits, can I safely store?

For example, I imagine this would not be safe enough:

5555 5555 555* 4444

Because you could calculate the missing digit.

Similarly, this would be safe, but not be as useful:

5555 5*** **** ****

Is there a well accepted pattern for storing partial credit numbers?