1) In some operating system's file systems, a data file over a certain size is stored in several "chunks" or fragments rather than in a single contiguous sequence of bits in one place on the storage medium, a process that is called fragmentation. This allows small unused sections of storage (for example, where old data has been deleted) to be reused. In the Windows 95 system, for example, files are fragmented and the location of the fragments for each file is kept in a file allocation table (FAT).
Because fragmentation over time can slow data access (each fragment of a file must be accessed for the entire file to be read in), the user may be provided with a defragmenter utility so that the data on the storage medium can be reorganized. This process is called defragmentation.
2) In TCP/IP, fragmentation refers to the process of breaking packets into the smallest maximum size packet data unit (PDU) supported by any of the underlying networks. In the Open Systems Interconnection OSI) reference model, this process is known as segmentation.