Sequential Numbering

Hi
I need to generate a UNIQUE alphanumeric number sequence (with the emphasis on unique) for an on pack, laser coding, phone in competition. The number sequence needs to be 6 characters long. Initially I was looking at creating a random alphanumeric number sequence, but, from what I can conclude, doing it this way might lead to number duplication and if we do that with a large prize winner we would have big issues. Bearing this in mind, I thought sequential numbering was the better way to go. How do I do this in Excel? (From what I can see, this gives me 6 Billion options). The alternative is going 5 character long which will give me 60 million options or, as a last resort, 4 characters long which will give me 1.6 million options.
Thanks
Peter

Thanks for your reply. I was looking at in excess of 30 million cells - if that is possible.

Thank you for your feedback and advising me of Excel row limitation. This in itself is not a problem, because it would not be a proble breaking the groups up into a million at a time. In actual fact, each production run is only about quarter of a million, so even batches of 250 000 would not be a problem.

You have resolved my problem - thank you.