s points where gateways, fire- walls or NATs may be present. However, we do not guaranteed that IP packets from one IP addressing domain can be directly routed to any interconnected IP addressing domain. Instead inter domain traffic will most likely be handled via firewalls or tunnels. Therefore, different IP addressing domains can have different (and possibly overlap- ping) address spaces [7]. Figure 9.9 illustrates the IP addressing domains involved in PS domain and IP subsystem services. UMTS allows usage of different IP addressing domains as shown in Figure 9.9; none- theless, it is possible that several different IP addressing domains come under a com- mon management. Hence, we can physically implement the different IP addressing do- mains as a single domain. The UMTS Network and Radio Access Technology Home Network IP Addressing domain IM Subsystem Home Network Inter-Network PS Domain Backbone IM Backbone Inter-PLMN Visited Network Backbone IM Subsystem Visited Network PS Domain Backbone Internet Traffic tunnelled over GPRS Intranets Gi Implemented on VPN or dedicated resources for each instance Figure 9.9 IP addressing domains involved in PS domain and IM services [7]. 9.5.2 Addressing and Routing to Access IM-Subsystem Services When a UE gets access to IM subsystem services, an IP address is required, which is logically part of the visited network IM subsystem IP addressing domain. We estab- lished this address using an appropriate PDP context, and for routing efficiency this context gets connected though a GGSN in the visited network. Figure 9.10 illustrates the connection between the UE and the visited network IM subsystem. Home Network IM Subsystem Virtual presence of UE in visited network IM subsystem (UE's IPaddress is here) Inter-Network IM Backbone Visited Network IM Subsystem Visited Network Internet PDP Context Intranets Figure 9.10 UE accessing IM subsystem services in the visited network. Towards IP Based Networks 9.5.3 Context Activation and Registration An IP address allocated to a UE either 