Ns and its components are available for free download from its web site.
For details about the ns copyright, please consult the file COPYRIGHTS in the root directory of the ns source code. (or see a version via cvsweb).
In general, ns use for research and education is supported. Use for internal design in commercial settings is supported as part of research. For other uses, please contact the ns developers (via ns-developers mail to isi.edu).