Stop Firewall for 12c grid control

Make sure that no firewall is enabled. It is usually enabled by default. In Redhat or Fedora, you can quickly disable the firewall:

First, login to the Linux computer.In a terminal window or shell prompt, type

 service iptables stop 

 If you see the below error while opening the 12c grid control EM url then

 stop the iptables

The connection has timed out
The server at ***** is taking too long to respond.
The site could be temporarily unavailable or too busy. Try again in a few moments.

If you are unable to load any pages, check your computer's network  connection.

If your computer or network is protected by a firewall or proxy, make sure 

that Firefox is permitted to access the Web.