Question about serials (NO, I'm not asking for any)

[nittro13]

I was just wondering how software companies develop their serials, so that people couldn't use ie. a keygen.

And also it seems that there was a pause with games using serials as copy protection. But now they seem to be back in style.

Is one possible solution having certain cd-groups that are printed to have a certain serials, or is this too expensive ?

I guess having a unique serial for each cd is impossible.

Any information is appreciated.