he IETF, and for 3GPP networks in particular. In the 3GPP networks there are two ways to provide UEs with dual-stack connectivity: two PDN Connections (PDP Contexts or EPS Bearers) - one for IPv4 and one for IPv6, or a single dual-stack IPv4v6 type of PDN Connection. Figure 5.15 illustrates both these setups, with parallel IPv4 and IPv6 PDN Connections above and single IPv4v6 PDN Connection below. Prior to Release-8, 3GPP supported only separate PDN Connections for IPv4 and IPv6. Dual-stack bearer type was introduced into the EPS architecture in Release-8, but for GPRS the support only came in Release-9. PDN Connections, be they PDP Contexts or EPS Bearers, require state in the networks, SO using a single connection burdens the network much less than two connections. Therefore, it is advisable to use the dual- stack PDN Connection. However, due to possible limitations of continuing to use the IPv6 Transition Mechanisms for 3GPP Networks IPv4v6 PDN Connection type in the networks, two parallel connections are needed in fallback cases. These fallback cases are needed because there is uncertainty over the support of the dual-stack PDN Connection in the networks. As described in depth in Section 4.5, this causes a set of issues and complications in PDN Connection setup procedures. The dual-stack approach obviously does nothing for the IPv4 address exhaustion prob- lem, and hence the dual-stack cannot really be a long-term solution. In the short term, IPv4 address shortage is managed by implementing IPv4 NAT functions and providing UEs with, possibly overlapping, private IPv4 addresses from the space allocated by RFC 1918 [25]. 5.4.3 IPv6-only Until most, if not all, Internet services have transitioned to IPv6, the strictly IPv6-only scenarios with no access to the IPv4 portion of the Internet are only applicable in limited deployments. The possible use cases include applications where the usage is restricted to a limited set of well-managed applications. Such use cases include, Machine-to-Machine (M2M) applications w