A Floating IP is a public IP which exists independently from your virtual machines. It can be dynamically associated to one of them, or disassociated and kept for later.

Create a Floating IP

In the Openstack Horizon interface, go to Network → Floating IPs and click the button "Allocate  IP to Project"

You can choose a Description, but it is not required.

The new Floating IP will then appear in the list:

Associate the Floating IP with an existing instance

On the Floating IP, select the "Associate" button, and choose the port of the instance you wish to assign the Floating IP to.

You can also reach this dialog from the instance, by choosing the menu "Associate Floating IP"

After pressing "Associate", you will be able to reach this instance through the public IP.

Disassociate the Floating IP

To detach the Floating IP from the instance, select either the button "Disassociate" from the Floating IP, or "Disassociate Floating" from the instance:

If you leave "Release Floating IP" unchecked, the Floating IP will still be in your project. You will be able to re-associate it later with an instance.

If you check "Release Floating IP", it will be removed from your project, and returned back to the pool of external IPs. If you create a new Floating IP, you will likely get a different one.