Well, next time you do an online install you can edit /etc/yum.conf :
from : keepcache=0
to : keepcache=1
..... before you start with # yum install <ns2-prerequisites>.
The packages are then saved in /var/cache/yum/<arch>/<version>/fedora/,
/var/cache/yum/<arch>/<version>/updates/
Or you can download the packages from Fedora, and install with
# rpm -Uvh <packages>.rpm
Releases
http://dl.fedoraproject.org/pub/fedora/linux/releases/
Updates
http://dl.fedoraproject.org/pub/fedora/linux/updates/
Required packages with dependencies, Fedora 17 example :
autoconf-2.68-3.fc17
automake-1.11.3-1.fc17
binutils-2.22.52.0.1-10.fc17
cloog-ppl-0.15.11-3.fc17.1
compat-gcc-34-3.4.6-24.fc17
compat-gcc-34-c++-3.4.6-24.fc17
cpp-4.7.0-5.fc17
gcc-4.7.0-5.fc17
gcc-c++-4.7.0-5.fc17
glibc-devel-2.15-37.fc17
glibc-headers-2.15-37.fc17
kernel-headers-3.3.7-1.fc17
libICE-devel-1.0.8-1.fc17
libmpc-0.9-2.fc17.2
libSM-devel-1.2.1-1.fc17
libstdc++-devel-4.7.0-5.fc17
libtool-2.4.2-3.fc17
libX11-devel-1.4.99.901-2.fc17
libXau-devel-1.0.6-3.fc17
libxcb-devel-1.8.1-1.fc17
libXext-devel-1.3.1-1.fc17
libXmu-devel-1.1.0-3.fc17
libXt-devel-1.1.2-2.fc17
patch-2.6.1-12.fc17
ppl-0.11.2-8.fc17
ppl-pwl-0.11.2-8.fc17
xorg-x11-proto-devel-7.6-21.fc17
.. And better get a copy of 'glibc' update version too, if present.
Required if you have downloaded e.g. 'glibc-devel' update version.
.