RD-IPtech SoftBank IP Infusion Cisco This document specifies an automatic tunneling mechanism for providing IPv4 connectivity service to end users over a service provider's IPv6 network. During the long transition period from IPv4 to IPv6-only, a service provider's network will have to support IPv6, but will also have to maintain some IPv4 connectivity for a number of customers, for both outgoing and incoming connections, and for both exclusive and shared IPv4 addresses. The 4rd solution (IPv4 Residual Deployment) is designed as a lightweight solution for this. In some scenarios, 4rd can dispense ISPs from supporting any NAT in their networks. In some others it can be used in parallel with NAT- based solutions such as DS-lite and/or NAT64/DNS4.