There are many ways to create numbers for identifying objects in this world.
All the different methods have their own specific quality.
One of the very best things in this area today is the globally unique identifier or GUID to provide a unique reference number.
Typically a GUID is represented as text and can be stored into a tag-field like other text strings.
The big advantage is, that this number has no "physical relation" to the data in the file, that means, the music content can be changed, the tag-field content can be changed, the filename can be changed, but the GUID will still the same for this file object, assuming that no one touched the GUID itself.
See also:
http://en.wikipedia.org/wiki/Globally_unique_identifier
Beside this GUID identifying method there exist some checksumming methods, which can help to detect if an data object has been changed anyway (size, content).
These are cryptographic hash functions like simple CRC32 or better MD5 or SHA-1 and so on.
Such a tool creates also an unique number for the one object, but the disadvantage is, that the calculation of the checksum costs time. The calculation has to be renewed every time, when the content has been changed anyway.
See also:
http://en.wikipedia.org/wiki/Md5
http://en.wikipedia.org/wiki/SHA-1
A GUID can be generated very easily and very fast.
There exist web applications, that can deliver one or many GUID.
There exist stand alone GUID generators. Each modern Windows system has a GUID generator built in.
Some days ago I made a proposal to use GUIDs also for the work on files with Mp3tag.
/t/11520/1
You can download a free GUID generator component and a generator user interface from there ...
https://onedrive.live.com/?cid=387b58366897...AAKAJNhZDtZRWYI
Once you got the software installed and running, you can create a GUID.
Copy the GUID to the clipboard. Create a new tag-field "MY_GUID" or so, fill in the value from the clipboard, save the change. Your file is now globally unique numbered.
For applying many GUIDs at once you can put all the created different GUIDs into a text file.
Use this text file as input for the Mp3tag converter "Textfile - Tag".
Fill the GUIDs into a tag-field "MY_GUID" or so.
After tagging each of the touched files carries a globally unique number.
Keep in mind:
If you need to recalculate an ID number each time when you see the file, you will wasting time for the calculating process. And if you do not use a "strong" checksum or encryption algorithm, then you have the chance to get a duplicate number.
DD.20110206.1749.CET
Edit. New link to "20110206.GUID.Generator.101.zip"
DD.20110206.2028.CET
Edit. New link to "HTA.GUID.Generator"
DD.20150724.0948.CEST