You are obviously not using IPv6, which is typically configured to do stateless autoconfiguration. The error you received "no IPv6 routers present" often means that you do not have an IPv6 network or router, or the physical link is down, in which case IPv4 will fail too. IPv6 and IPv4 are separate issues. If you are not using IPv6, errors about it can be ignored, or best IPv6 should be disabled.
Perhaps you are confusing the task of the network manager with the task to configure your TCP/IP network. The NetworkManager gnome applet will connect any network device when a connection for that device becomes available. It is useful for user Desktop or Laptop systems where automatic setups are desired, for instance, to automatically have the system switch between WIFI and wired connections..
The Network Manager It is typically not useful for server installations, which is the primary focus of OL or RHEL. It can interfere with your intended network setup. Of course, if your TCP/IP configuration is incorrect, the NetworkManager can help to establish a network connection.
I doubted it, but I wondered if my server's hardware suddenly changed.
Other Linux distributions installed perfectly fine, and started networking with no intervention on my part.
And previous installs of Redhat 5.5, and Windows booted and networked fine.
Here is the secret.
After installing Redhat 6.4 yet another time, I noticed an icon in the lower right hand corner of the screen.
It was making either a clockwise, or counter clockwise motion.
When I clicked it, there was a pop up menu.
One option said: Enable network
Turns out that Redhat 6.4 does not automatically enable the network by default !!!!
What you are playing with is the NetworkManager gnome applet, which only works under the Gnome Desktop and only after you have logged into a Gnome session.
When you install the OS, the anaconda installer provides a "connect automatically" checkbox in the network device configuration screen. You most likely missed it and as a result it does not set the "ONBOOT=yes" parameter in the ifcfg-eth0 network script, which won't "UP" the device after a system restart. As far as I remember, this was the same behavior in the previous OS release.
The gnome NetworkManager applet serves a different purpose, like I previously tried to explain.