Network operating systems (NOSs) are OSs that exist mostly or fully for, which 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 make little sense without their network functions. Many OSs have some networking ability, but not all such OSs are NOSs.

On this page, OSs are arranged in three groups and levels: 1) Top group: types or classes of OS. 2) Middle group: OSs for which there are more than one instance of an OS of this name/type, an OS family. 3) Bottom group: specific OSs, individual instances; there is only one OS of this name/type.