s, a user can keep the same IP address, stay connected, and maintain ongoing applications while roaming between IP networks. When a user leaves the network with which his or her device is associated (i.e., the home network) and enters the domain of a foreign network, the foreign network uses the Mobile IP protocol to inform the home network of a care-of address to which all Chapter 16 Emerging Wireless Applications packets for the user's device should be sent. The way it works can be illustrated through a post office analogy. Let's say I move from San Francisco to Hong Kong. When I get to Hong Kong, I drop off a note at the Hong Kong post office with my new Hong Kong address. The Hong Kong post office then notifies the San Fran- cisco office of my new address. When the San Francisco post office receives mail for me, it knows to forward it to my Hong Kong address. Mobile IP is highly desir- able in wireless WAN environments where users need to carry their mobile devices across multiple LANs that have different IP addresses. Mobile IP is the underlying technology for support of various mobile data and wireless networking applications. Mobile IP technology is embedded in the func- tionality of packet equipment for 2.5G and 3G networks. For example, General Packet Radio Service (GPRS) depends on Mobile IP to enable the relay of messages to a GPRS phone via the serving GPRS support node (SGSN) from the gateway GPRS support node (GGSN) without the sender needing to know the serving node IP address. (GPRS is discussed in Chapter 14.) In addition, Mobile IP enables advanced applications such as unified messaging. As shown in Figure 16.1, there are several key elements to Mobile IP: Mobile node-A mobile node is a device capable of performing network roaming. Home agent-A home agent is a router on the home network that serves as the point for communications with the mobile node. Foreign agent-A foreign agent is a router that functions as the mobile node's point of attachment when it travels to the foreign network. Care-of 