Localhost vs. 0.0.0.0

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I've had problems troubleshooting IP addresses with my setup and want to verify some questions, if you could help:

1. Should I use localhost or 0.0.0.0 when referring to myself, or does it have to be a real URI?

2. I have read that if you don't have a DNS name, you must use ROS_IP and not ROS_HOSTNAME, yet I see many examples where an ip address is used for ROS_HOSTNAME. For example, here: http://emanual.robotis.com/docs/en/pl... near the end you will see ROS_HOSTNAME=192.168.9.100

3. Can I omit both ROS_IP and ROS_HOSTNAME usually - unless I have more than one network adapter. If I have more than one then by default ROS would use all of them. But by specifying ROS_IP or ROS_HOSTNAME I can limit ROS to a specific one.

Followup

1. On the computer running ROSCORE, I understand that this is almost always sufficient.

export ROS_MASTER_URI=http://localhost:11311

Under what special circumstances does that computer need a specific IP address of ROS_MASTER? And does it ever need ROS_IP or ROS_HOSTNAME?

Thanks!