ns-3 is a discrete-event network simulator oriented towards network research and education, with a special focus on Internet-based systems. The ns-3 project is designing a follow-on successor to the popular ns-2 simulator.
In ns-2, simulation scripts are written in OTcl. In ns-3, simulation scripts are written in C++, with support for extensions that allow simulation scripts to be written in Python. These Python bindings have yet to be written, but the goal is for full or near-full API support at the Python level.
ns-3 is intended to provide better support than in ns-2 for the following items:
ns-3 is a rewrite of the core of the simulator. ns-2 does not presently run in ns-3, although we are studying approaches to allow ns-2 to be run as part of ns-3, as well as studying which models will be ported from ns-2 to work natively in ns-3.