GUID – Explained Clearly

There are multiple GUID generation algorithms, but I’ll pick one of them for concreteness, specifically the version described in this Internet draft.

The first 60 bits of the GUID encode a timestamp, the precise format of which is not important.

The next four bits are always 0001, which identify that this GUID was generated by “algorithm 1”. The version field is necessary to ensure that two GUID generation algorithms do not accidentally generate the same GUID. The algorithms are designed so that a particular algorithm doesn’t generate the same GUID twice, but without a version field, there would be no way to ensure that some other algorithm wouldn’t generate the same GUID by some systematic collision.

The next 14 bits are “emergency uniquifier bits”; we’ll look at them later, because they are the ones that fine tune the overall algorithm.

The next two bits are reserved and fixed at 01.

The last 48 bits are the unique address of the computer’s network card. If the computer does not have a network card, set the top bit and use a random number generator for the other 47. No valid network card will have the top bit set in its address, so there is no possibility that a GUID generated from a computer without a network card will accidentally collide with a GUID generated from a computer with a network card.

Once you take it apart, the bits of the GUID break down like this:

  • 60 bits of timestamp,
  • 48 bits of computer identifier,
  • 14 bits of uniquifier, and
  • six bits are fixed,

for a total of 128 bits.

The goal of this algorithm is to use the combination of time and location (“space-time coordinates” for the relativity geeks out there) as the uniqueness key. However, timekeeping is not perfect, so there’s a possibility that, for example, two GUIDs are generated in rapid succession from the same machine, so close to each other in time that the timestamp would be the same. That’s where the uniquifier comes in. When time appears to have stood still (if two requests for a GUID are made in rapid succession) or gone backward (if the system clock is set to a new time earlier than what it was), the uniquifier is incremented so that GUIDs generated from the “second time it was five o’clock” don’t collide with those generated “the first time it was five o’clock”.

Raymond Chen, devblogs.microsoft.com