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
If more help is needed then please explain the problem more completely and provide more information about your system by running "./getinfo.sh network" and showing us the output file.
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