CentOS 6 start network connection on boot

Unlike CentOS 5, CentOS 6 desktop install does not configure network devices to start on boot.
Assuming that your main wired connection is eth0...

Edit /etc/sysconfig/network-scripts/ifcfg-eth0 and change the lines

NM_CONTROLLED=”yes”
ONBOOT=”no”

to

NM_CONTROLLED=”no”
ONBOOT=”yes”

The same principle applies to other network devices.
If you make other changes to these files besides the start behaviour, run service network restart so it can take effect.