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Obtaining persistent IP addresses to your instances

It is possible to create a port without creating an instance. When you create a port in such a manner, the port will be allocated IP address(es) from the network you specify just like when you create an instance. Afterwards, when creating an instance, you can specify to attach the port you created as an interface to the instance.

When ports are created upfront of instance creation and then attached, the ports will not automatically be deleted when those instances are deleted nor if you detach that interface/port from an instance. Thus, the IP address(es) will still be allocated to that port and it can be attached to another instance that you want to have the same IP address as the one you deleted. This can be useful if you want to change flavor of an instance by snapshotting an old instance and creating a new one with a different flavor from that snapshot.

If you however create the instance without creating a port for it up front, the interface/port will be deleted and the IP address will go back to the pool of available addresses when the instance is deleted or the port is detached from the instance.

Important note

Even if you create one or more ports/interfaces you must only connect one port/interface to each instance.

It is currently not possible to create a port through the GUI, so it needs to be done through the API directly or with tools utilizing the API, like the Openstack CLI or Terraform for instance.

Here is an example using the Openstack CLI: 

$ openstack port create --network private dummytest
$ openstack port list |grep dummy
| aead4548-e41d-4724-acd5-6aeb8d9fbce8 | dummytest  | fa:16:3e:d5:bc:ef | ip_address='10.68.129.21', subnet_id='059d94a0-0fc1-40dd-9814-eb00571c6a4d'           | DOWN   |

When the port is created you can choose to attach to it instead of a network when launching a new instance in the GUI. Just skip the network part, and select the port in the next step "Network ports". Now the network port and the instance can be maintained in separate lifecycles.

It is of course possible to create an instance through the Openstack CLI without attaching to a network, and then to attache the port later on, or create it by specifying the port name like this:

$ os server create --image ubuntu-20.04 --flavor l2.c2r4.100 --port dummytest my-instance