You may have trouble accessing newly-launched instances. This is because all inbound flows are blocked by default for optimal security, and for this reason, instances do not respond to connection requests such as ping, telnet, or ssh. The following error message is returned:
ssh: connect to host xxx.xx.xx.xxx
port 22: Operation timed out.
To access an instance, you thus need to add a rule to its security groups to authorize the connection. For more information, see Accessing Your Instances.