The 4rd automatic tunneling mechanism makes IPv4 Residual Deployment possible via IPv6 networks without maintaining for this per-customer states in 4rd-capable nodes (reverse of the IPv6 Rapid Deployment of 6rd). To cope with the IPv4 address shortage, customers can be assigned IPv4 addresses with restricted port sets. In some scenarios, 4rd-capable customer nodes can exchange packets of their IPv4-only applications via stateful NAT64s that are upgraded to support 4rd tunnels (in addition to their IP/ICMP translation of [RFC6145]).