ed tunneling and SGW in the case of PMIPv6-based tunneling. It is quite common that multiple IPv4 enabled APNs have overlapping address spaces, especially for big operators. Naturally, when the user plane IP traffic gets routed to (S)Gi-interface, care needs to be taken if overlapping address IPv6 in 3GPP Networks HLR/HSS (Static addressing per APN per subscriber) Gr/S6a/S6d ((Q)) GGSN/PGW SGSN/MME (S)Gi Operator's Internal Gn/S5 Pools DHCP Server Pools External Client/ Relay/ (Internet) Server AAA Server Pools Client Addressing scope Figure 4.19 IP address management methods and places for transparent access case. spaces are in use. Techniques vary, but most generally known techniques used in fixed IP networking to separate traffic are available on the (S)Gi-interface as well. The IP addressing is managed either by the operator (i.e. the transparent access to Inter- net) or by an external PDN or ISP (i.e. the non-transparent access to an intranet or ISP). These two different deployment models were discussed in Section 4.2.4. As a reminder, Figure 4.19 shows an example architecture where the IP addressing is managed by an oper- ator and the IP address space topologically belongs to the operator's network. Figure 4.20 shows an example architecture where the IP addressing is managed by an external PDN or an ISP, and also topologically belongs to the external network. Technically the line between the IP addressing and the UE authentication mechanisms in transparent and non- transparent cases is subtle. A thumb of rule is that in the non-transparent case, the IP address space does not topologically belong to the operator's network (where the PGW is located) and some kind of tunneling (either at layer 2 or layer 3) is involved in routing IP traffic to the PGW from the external network. Table 4.5 lists typical places where IPv4 addresses and IPv6 prefixes are managed in 3GPP deployments. The following sections go into further details of how majority of the IP address con- figuration mechanisms listed in Table 4.4 work.