CentOS-6 uses NetworkManager by default. If you wish to use the "traditional" network service from the command line then use chkconfig to disable NetworkManager and make sure the network service is running. If etc/sysconfig/network-scripts/ifcfg-eth0 (or whatever your device is named) has a line
Just wanted to post because this helped me. I'm running CentOS 7. For my version, the path is correct but eth0 was named and I had an option of ONBOOT=NO, set it to YES and this set my NIC as active on boot.