HFS Plus allocates space in units called allocation blocks; an allocation block is simply a group of consecutive bytes. The size (in bytes) of an allocation block is a power of two, greater than or equal to 512, which is set when the volume is initialized. This value cannot be easily changed without reinitializing the volume. Allocation blocks are identified by a 32-bit allocation block number, so there can be at most 232 allocation blocks on a volume. Current implementations of the file system are optimized for 4K allocation blocks.