[QUOTE=yunchiyufu;2254496]Is there some number or identifier that can be used to recognize each DVD /CD ?[/QUOTE]
Each DVD / CD of what? Each that you own? Each that you and your friends own? Each that has been manufactured ever?
You can come pretty close to the first two subgroups for recorded media.
In Windows, a volume serial number is synthesized from the date of the contents of the CD /DVD. That works pretty well – haven’t seen a collision yet for about 1800 CD/DVD. (Alternatively, if you extract the ‘volume creation date’ from the medium, that would be even better, as it hasn’t been folded into a single 32-bit value.)
For ISO9660-structured media, you can do still somewhat better, I think, by collecting all data in the volume descriptor set (i.e. from logical sector 16 until you hit the appropriate ‘end’ block), and SHA1-hash the lot. That will include the creation timestamps, the volume names, the sizes of the volumes, root directory entry, and various other things.
Still, it won’t guarantee absence of collisions, but it will take some pretty unusal circumstances to cause a collision. Like: create a .ISO image from whatever CD you want to collide with. Change one single byte in logical sector 0. But then it would take a full all-user-data hash to see the difference, and that takes time.