 IPv4 over IPv6 as the encapsulation mode. A generic network topology for mapped address and port number deployments is shown in Figure 5.11. MAP specifies an algorithm to construct an IPv6 address from an IPv4 address and a port range. Figure 5.12 shows the construction of the MAP IPv6 address [21]. The figure contains the Basic Mapping Rule (BMR), a general presentation of the mapping scheme, and the Forwarding Mapping Rule (FMR) for both translation and encapsulation modes. It also has the Default Mapping Rule (DMR) for the translation mode. In the case of encapsulation mode DMR the IPv6 address would just be a normal (anycast) address of the BR. In the case of the FMR the IID contains the (public) IPv4 address of the CPE or the intended destination and the used Port-set Identifier (PSID). For the actual algorithm of how to derive the Embedded Address (EA) bits and the PSID, please refer to Section 5 of [21]. The port set calculation algorithm allows automatic exclusion of the low range ports, for example, ports from 0 to 1023. Depending on provisioning each shared address could, for example, be allocated 1024 ports, which would end up to 1/63 address sharing ratio (when the low ports are excluded). As a reminder, IPv6 Transition Mechanisms for 3GPP Networks IPv6 Prefix_A IPv4 Address_A MAP CPE #1 Ports 4096-5119 Border Relay/Router (anycast reachable) Private IPv6 only IPv4 and backbone MAP CPE #2 network (MAP Intenet domain) Border Relay/Router Private (anycast reachable) CPEs provisioned with a shared IPv6 Prefix_B public IPv4 address and a port IPv4 Address_A range; and a unique IPv6 prefix.. Ports 5120-6143 Figure 5.11 Network topology for a generic mapped address and port deployments. there are three different possibilities how to construct the EA bits (we shortcut a bit and discard the operator set prefix part for the EA bits): 1. It contains a complete IPv4 address, that is no address sharing is in use. 2. It contains a suffix of a shared IPv4 address and PSID for identifying the set of ports out of to