Distributed operating systems (OSs) are a type of network operating system (NOS). NOSs exist mostly or fully for, their main reason for being, is to facilitate networking, between two or more computers, to operate and improve networks, with non-trivial networking (e.g., routing) included and/or built in, which need not be added later, and they make little sense without their network functions. Many OSs have some networking ability, but not all such OSs are NOSs. Distributed OSs go beyond most NOSs, to divide, distribute, and even dynamically migrate tasks and information, to operate, over networks to run on two or more processors, often called clustering, and heterogeneous multiprocessing. By definition, distributed OSs are multiprocessor OSs, usually asymmetric. On this page, OSs are arranged in two groups and levels: 1) Top group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 2) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.