In case you ever need to install by yum something local (from a CD for instance in case you don’t have Internet access and you need to compile the ethernet card’s loadable module) you may use this:
yum –disablerepo \* enablerepo CentOS install gcc
In the example above /dev/scd0 (there’s a CentOS distro DVD on the serial optical device) is mounted as /media/CentOS and the command will install gcc.