ROS_IP and ROS_HOSTNAME

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Hello,

In my understanding according to the wiki, ROS_IP is for the IP address of the host machine where a ROS node runs and ROS_HOSTNAME is for a resolvable host name. But I'm confused because an IP address is also a resolvable name(maybe resolved name?). So even when I set an IP address to ROS_HOSTNAME, everything works well.

I saw the source codes and found that both are treated almost same(rocpp, rospy).

So my questions are

1. Why are there two environmental variablesROS_IP and ROS_HOSTNAME, even though both are treated almost same?
2. Is it better to always use ROS_HOSTNAME to avoid network troubles? I've seen some people have troubles because they set both variables and ROS_IP doesn't work as they expected.

Thanks in advance.