...

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"

...

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.

...

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:

...