Comment 2 for bug 1499670

Theoretically this can be implemented.

What we need to do are:
- to retrieve a list of routers to gather router interfaces
- to retrieve a list of router interfaces to calculate subnets reachable from routers connected to external network(s)
- to retrieve a list of ports attached to all servers displayed in the instance table
- to calculate which ports belong to the reachable subnets to determine which instances can be associated with FIP

To do this, we need a full list of routers and a full list of ports (of the scoped project).

This might cost and we need to consider a balance between usability and performance.