After a new install of CentOS 6, I noticed that there was no network connection active.

ifconfig -a showed eth0 and lo, but eth0 had no IP attached to it.

The fix:

cd /etc/sysconfig/network-scripts

Edit:

vi ifcfg-eth0

Find:
ONBOOT=no

Change to:
ONBOOT=yes

Save ifcfg-eth0

Reset eth0:

ifdown eth0
ifup eth0

Test with a ping or just run ifconfig -a to see if the IP was assigned.