The default behavior of the Neutron router is to source NAT traffic from instances that lack floating IPs when traffic egresses the external, or gateway, interface of the router.

Performing a packet capture on the eth2.43 interface of the controller node that corresponds with the PROJECT_NET network, we can observe ICMP traffic from the instances sourcing from their real or fixed addresses as the traffic heads towards the router. The reply also references the same fixed IP address:

From the eth2.30 interface on the controller node that corresponds to the GATEWAY_NET network, we can observe ICMP traffic from the instances after it has traversed the router sourcing as the router's external address, 10.30.0.106:

A look at the iptables chains within the router namespace reveals the NAT rules responsible for this behavior:

In this configuration, instances can communicate with outside resources through the router as long as the instances initiate the connection. Outside resources cannot initiate connections directly to instances via their fixed IP address.