I need a static IP for my VM

To reach a VM you need to assign a floating IP to it. Floating IPs have some remarkable characteristics:

  1. Once you select a floating IP from the pool this IP stays assigned to you until you return it to the pool.
    This is very helpful if you want to register this IP with remote services (such as NFS exports), because until you explicitly return it to the pool you can be sure that this IP is only used by you.
  2. Once you assign a floating IP to a particular VM this IP stays assigned to it even if you shelve your VM.
    This ensures that a VM will use the same IP even if you shelve it inbetween. To free the floating IP you need to explicitly disassociate it or delete the VM.