See GUIDs are globally unique, but substrings of GUIDs aren't.
Hashing a GUID may work (but I'm not sure), but a better idea would be to come up with your own algorithm inspired by the GUID algorithm and based on the constraints of your system that you are aware of, for example:
- If you know that you will only be generating IDs on a handful of systems then the MAC address portion of the GUID can be shortened.
- Similarly if you assume that your app will be long gone in 100 years you can shorten the timstamp portion
- If you only have one algorithm then there is no need for the algorithm identifier
- If you also know that you wont be generating them too quickly you can shorten the "emergency uniquifier bits"
