A common question in OS interview questions, don't miss this one. There are two types of fragmentation i.e. Internal fragmentation and External Fragmentation.  Internal Fragmentation: Internal fragmentation occurs when memory is partitioned into mounted-sized blocks. When a memory method is requested, the mounted-sized block is assigned to the method. When the memory allotted to the method is slightly larger than the memory requested, the difference is referred to as internal fragmentation.  External Fragmentation: External fragmentation occurs when there is sufficient space in memory to satisfy a method's memory request. However, the process's memory request cannot be met because the memory provided is not contiguous. External fragmentation will occur whether you use a first-fit or best-fit memory allocation strategy.