Versioning. The 4 says it is "version 4" in the RFC 4122 taxonomy, and the fact that the first two bits of y are "10" is meant to indicate that this is a UUID following that standard. Certain Microsoft GUIDs start with "110" in that field, and apparently there's another legacy format (Network Computing System) characterized by the first bit of y being "0". Therefore, an RFC 4122 compliant UUID will not overlap with either of these namespaces.