ition tool for IPv6 transition is the dual-stack as defined in RFC 4213 [2]. This approach, when referred to as a transition tool, not only means that the IP stack has Deploying IPv6 in 3GPP Networks implementation for both IPv4 and IPv6, but that the stack is actually using both of them at the same time and in parallel (instead of, for example, tunneling IPv4 over IPv6). Both address families use independent means for address configuration, for example IPv4 can use Dynamic Host Configuration Protocol (DHCP) or link-layer means, and IPv6 can use Stateless Address Autoconfiguration (SLAAC) or Dynamic Host Configuration Protocol version 6 (DHCPv6), when supported. In dual-stack cases hosts ask for both A and AAAA DNS resource records when performing name resolution (see more in Section 3.10.2). We highlight this because performing both queries efficiently is an important feature to save time in connection establishments. Furthermore, in the case of single address family connections, a host might optimize and only send out DNS queries for the addresses that can actually be used for communications. IPv4 Addressing Issues Nowadays the dual-stack approach is often - and will be even more in the future - challenged by the shortage of IPv4 addresses. Therefore, the IPv4 addresses are increasingly network address translated, i.e. assigned to hosts from the private IPv4 address space allocated in RFC 1918 [25] (i.e. addresses from spaces 10/8, 172.16/12, or 92.168/16). In 3GPP networks it is also possible to use an approach called deferred IPv4 address allocation to mitigate load for an operator's IPv4 address pools (see Sections 4.4 and 4.4.4). As a result of NAT'ed IPv4 address, the hosts using dual-stack access do not typically get the same level of service with both address families. The IPv4 connections may need more frequent keep-alive traffic than IPv6, and IPv4 is more likely to not allow incoming connections due to NAT, while IPv6 does not always come with a firewall. Address Family Performance Differences While t