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. This category lists sites for NOSs that are available under an Open Source license.