Install information is here
Automatically : Please define automatically.
Only basic software is installed at OS install,
+ the groups of packages you select.
Installing more packages after the OS install :
All those are to be installed with # yum,
from the repositories.
Which contain say 15,000 packages.
The other 50,000 very specialized pieces of software,
like ns2 : Are still to be installed manually until you
or somebody else make up an rpm package. Which can then
be installed with : # rpm -UVh <package>.
Or # yum localinstall <package>.
People use all the different special versions of ns2,
which version do you want ?