When I used "rosnode info turtlesim", I saw:

...

contacting node http://i7:56977/ ...

ERROR: Communication with node[http://i7:56977/] failed!

I thought maybe it was because I did not have $ROS_HOSTNAME defined as described here: http://www.ros.org/wiki/ROS/NetworkSetup#Single_machine_configuration

So I added export ROS_HOSTNAME=localhost

After that "rosnode info turtlesim" give me:

...

contacting node http://localhost:60372/ ...

Pid: 6178

I thought it was strange to see a different port number, but rosnode list started working.

Then I went to the next tutorial and did "roscore" while other roscore was running.

Then I noticed that  "rosnode list" again doesn't work and "rosnode info turtlesim" now gives:

contacting node http://localhost:39889/ ...

ERROR: Communication with node[http://localhost:39889/] failed!