Mobile IP is an open standard, defined by the Internet Engineering Task Force (IETF) RFC 3220. By using Mobile IP, you can keep the same IP address, stay connected, and maintain ongoing applications while roaming between IP networks. Mobile IP is scalable for the Internet because it is based on IP—any media that can support IP can support Mobile IP.

 The Cisco Mobile Networks feature enables a mobile access router and its subnets to be mobile and maintain all IP connectivity, transparent to the IP hosts connecting through this mobile access router.

 Currently, this feature is a static network implementation that supports stub routers only. In IP networks, routing is based on stationary IP addresses. A device on a network is reachable through normal IP routing by the IP address it is assigned on the network. When a device roams away from its home network, it is no longer reachable by using normal IP routing. This results in the active sessions of the device being terminated.

 Mobile IP enables users to keep the same IP address while travelling to a different network, ensuring that a roaming individual can continue communication without sessions or connections being dropped. Because the mobility functions of Mobile IP are performed at the network layer rather than the physical layer, the mobile device can span different types of wireless and wire line networks while maintaining connections. Remote login, remote printing, and file transfers are examples of applications where it is desirable not to interrupt communications while an individual roams across network boundaries.

 Also, certain network services, such as software licenses and access privileges, are based on IP addresses. Changing these IP addresses could compromise the network services. A device that can roam while appearing to a user to be at its home network is called a mobile node. Examples of mobile nodes include: a personal digital assistant, a laptop computer, or a data-ready cellular phone—that can change its point of attachment from one network or subnet to another.

This mobile node can travel from link to link and maintain communications using the same IP address. There is no need for any changes to applications, because the solution is at the network layer, which provides the transparent network mobility. The Cisco Mobile Networks feature comprises three components—the mobile access router (MR), home agent (HA), and foreign agent (FA). Figure shows the three components (mobile access router, home agent, and foreign agent) and their relationships within the mobile network.

The mobile access router functions similarly to the mobile node with one key difference—the mobile access router allows entire networks to roam. For example, an airplane with a mobile access router can fly around the world while passengers stay connected to the Internet. This communication is accomplished by Mobile IP aware routers tunnelling packets, which are destined to hosts on the mobile networks, to the location where the mobile access router is visiting.

 The mobile access router then forwards the packets to the destination device. These devices can be mobile nodes without Mobile IP client software. The mobile access router eliminates the need for a Mobile IP client. The mobile access router ―hides‖ the IP roaming from the local IP nodes so that the local nodes appear to be directly attached to the home network. A home agent is a router on the home network of the mobile access router. It provides the anchoring point for the mobile networks.

The home agent maintains an association between the home IP address of the mobile access router and its care-of address, which is the current location of the mobile access router on a foreign or visited network. The home agent is responsible for keeping track of where the mobile access router roams and tunnelling packets to the current location of the mobile network. The home agent also inserts the mobile networks into its routing table.

A foreign agent is a router on a foreign network that assists the mobile access router in informing its home agent of its current care-of address. It functions as the point of attachment to the mobile access router, delivering packets from the home agent to the mobile access router. The foreign agent is a fixed router with a direct logical connection to the mobile access router. The mobile access router and foreign agent need not be connected directly by a wireless link. For example, if the mobile access router is roaming, the connection between the foreign agent and mobile access router occurs on interfaces that are not on the same subnet. This feature does not add any new functionality to the foreign agent component.