If you have just set up a new Centos server and found that the network isn’t functioning, and you have no IP address for it then fear not, its a ‘secure’ default for Centos to not enable networking until you are ready to do so.
cd /etc/sysconfig/network-scripts/ sed -i -e 's@^ONBOOT="no@ONBOOT="yes@' ifcfg-eth0
If this doesn’t help, then double check the filename in that folder. You may have ifcfg-lo instead.
More help here: http://wiki.centos.org/FAQ/CentOS6#head-b67e85d98f0e9f1b599358105c551632c6ff7c90