the obvious disadvantage of this solution is that the scope is a single subnetwork. It is the responsibil- ity of the operating system to provide transparent layer two connections to higher layers of the protocol. Subnetwork mobility protocols provide mobility "invisibly." Unmodified network layer, transport layer, and application layer protocol entities continue to function without knowledge of the underlying link layer mechanisms. Subnetwork layer mobility is inherently supported by all wireless cellular systems, such as intra-cell mobility in a circuit-switched cellular network. Inter-cellular mobility in cellular networks is achieved with the help of sophis- ticated switching mechanisms in the mobile switching centers (MSCs) that control the base stations. The main objective of this type of mobility protocol is to provide limited mobility to subnetworks only, which can work in con- junction with higher-layer protocols to provide mobility outside the scope of a subnetwork. 17.3.2 Network Layer Network layer mobility finds its utility exactly where the subnetwork layer mobility falls short with respect to the scope of operation. The principal goal has been to provide network-level transparency. Specifically, in the Inter- net, hosts are identified solely with their network-level point of attachment (e.g., IP address). Thus, when a network layer mobility protocol is operat- ing in a Transmission Control Protocol (TCP)/IP-based network, the upper layers do not have to bother about IP address change due to host mobil- ity. Although the notion of network layer mobility started with the Mobile IP proposal [4], a host of different network layer mobility protocols have been proposed, each attempting to improve performance with respect to cer- tain parameters. These protocols can be broadly classified into two classes micro-mobility and macro-mobility-based on their scope of operation with respect to the administrative domains in the Internet. Micro-mobility proto- cols work within an administrative domain, while macro