ns-3 is described as 'Discrete-event network simulator for Internet systems, targeted primarily for research and educational use. ns-3 is free, open-source software, licensed under the GNU GPLv2 license, and maintained by a worldwide community' and is a network simulator in the network & admin category. There are 1 alternatives to ns-3 for Mac, Windows and Linux. The best ns-3 alternative is OMNeT++, which is both free and Open Source.
OMNeT++ is a modular, component-based C++ simulation library and framework, primarily for building network simulators. OMNeT++ can be used for free for non-commercial simulations like at academic institutions and for teaching.
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