As explained above, Microsoft Windows (and its predecessor, MS-DOS) allocates disk space for files in clusters. This allows the operating system to fragment a file if there is enough disk space for the file but not enough contiguous free space to store the file without fragmentation.
This type of fragmentation is known as external fragmentation. [Rul95]
In addition, as discussed earlier in the section on the FAT file system, files on the disk can suffer from internal fragmentation. If the size of a file is not a multiple of the size of a cluster, then part of the last cluster allocated to the file is empty wasted space since files can not share a cluster. [Rul95]