Excel Assistance Please

Hi. I need help in designing an excel spreadsheet. I'm not a uni student...I am trying to implement this at work. Here goes;

I have a list of 8 digit serial numbers in one column. In another column i have a list of assigned product names (which happen to be numbers). I need to assign a MAC address (in HEX format) to specific products that require them. The MAC address that is used should come from a list where the MAC addresses are listed numerically. In other words, we bought 2 blocks of MAC addresses from IEEE (that's a number of millions of MAC addresses) and there should never be a double-up.

Now I created such a file with relevant formulas so when i type in the next available serial number with a specific product, up pops a MAC address. If the product does not require a MAC address then it's blank. It works fine but the method is frought with danger. If someone decides to alter a previous entry or change the product name, all preceding assigned MAC addresses will change. It should not happen, but there is always that possibility currently.

I'm sure there is a better way to do this and am asking for your help. Any ideas?