despite changes in the link layer point of attachment. Thus, when a MS moves from a home network to a foreign network it registers with its Home Agent (HA) an IP address that the HA can use to tunnel packets to the MS (i.e. a Care Of Address (COA)). The HA intercepts packets addressed to the MS's home address and tunnels these packets to the COA. Here we do not require interaction with UMTS location registers. A COA may be, for example a dedicated address each MS gets in the visited network (co-located-COA); if so, the MS is the tunnel endpoint. Otherwise, the COA can be an address advertised (or retrieved) by a Foreign Agent (FA); if so, it is a FA-COA and the FA is the tunnel endpoint. The FA extracts packets from the tunnel and forwards them to the correct RAN logical link in order to deliver them to the appropriate MS. 9.10.2 Differences between IPv4 and IPv6 The key differences between protocols MIPv4 [9] and MIPv6 [10] can be summarized as follows [8]: Towards IP Based Networks Mobile IPv4 allows the use of Foreign Agents (FAs) to forward traffic thus requir- ing one care of address for multiple mobile stations, or the use of co-located care-of addresses (COA). In contrast MIPv6 supports co-located COAs only. MIPv4 has route optimization as an add-on, whereas it is an integral part of the MIPv6 specification. MIPv4 route optimization still requires traffic to be tunnelled between the corre- spondent host (CH) and the mobile station. In MIPv6 packets can be forwarded without tunnelling, i.e. only with the addition of a routing header. In MIPv4 the Home Agent (HA) must get involved in the set up of optimized routes. In MIPv6 the mobile station can initiate an optimized route to a CH directly (without involving the HA), and therefore more quickly and efficiently. In MIPv4 we obtain a COA from a FA or via DHCPv4. In MIPv6 we may obtain a COA via IPv6 stateless or state-full address auto-configuration mechanisms. In MIPv4 we require separate mobile IP specific messages to communicate with the FA, HA and CHs (whe