Please click the following link to download ns-3.28.1, released August 2018:

If you are already using ns-3.28 and wish to patch upgrade to ns-3.28.1, a patch is available here.

If you wish to download the initial ns-3.28 release from March 2018, that source code is available here:

A source code patch to update ns-3.27 release to ns-3.28 release is available here. Other patches to migrate older versions of ns-3 (back to ns-3.17) to the latest version can be found in the same directory; they must be applied sequentially to upgrade across multiple releases.