CentOS 6? If so I got burned by this one as well. Check out the official CentOS 6 FAQ.
The issue is that
NetworkManager is now responsible for the
eth0 device and by default it's disabled. During installation you can configure
eth0 so that it will start up automatically, but the default is that it doesn't.
If you need to configure this after you've setup the system you can follow the steps in the FAQ which tell you exactly how to setup the
The nuts of the fix are that you need to edit this
eth0 configuration file,
/etc/sysconfig/network-scripts/ifcfg-eth0, and make sure that the
ONBOOT line is enabled, i.e.,
Here's a couple of screenshots of where to configure this during the CentOS installation. I totally missed it the first couple of times.