You must have an active IBMid and a valid software maintenance or subscription agreement for AIX 7.3.
Licensing & support
You can legally download AIX 7.3 if you fall into one of these three categories: download aix 7.3 iso
The inclusion of a robust AIX Open Source Toolbox with over 800 packages (including Python 3.11 as default) makes it feel much more like a modern Linux environment for developers. You must have an active IBMid and a