Comment 0 for bug 1930499

Currently when adding an interface to a router, Horizon shows subnets with gateway ip.

On the other hand, Horizon supports attaching a subnet to a router with fixed interface ip.
In this case gateway ip is not required but a new port is created with a given ip in the subnet
and the port is attached to the router.
This is useful especially when users want to connect an instance to multiple subnets with a single default gateway (only one subnet has gateway ip while the other subnet don't).

However because of the current logic to filter out subnets without gateway ip, we can't add
a subnet without gateway ip to a router using fixed ip now.