Mobile IP is an Internet Engineering Task Force (IETF) standard communications protocol that is designed to allow mobile device users to move from one network to another while maintaining their permanent IP address. Defined in Request for Comments (RFC) 2002, Mobile IP is an enhancement of the Internet Protocol (IP) that adds mechanisms for forwarding Internet traffic to mobile devices (known as mobile nodes) when they are connecting through other than their home network.
In traditional IP routing, IP addresses represent a topology. Routing mechanisms rely on the assumption that each network node will always have the same point of attachment to the Internet, and that each node's IP address identifies the network link where it is connected. Core Internet routers look at the IP address prefix, which identifies a device's network. At the network level, routers look at the next few bits to identify the appropriate subnet. Finally, at the subnet level, routers look at the bits identifying a particular device. In this routing scheme, if you disconnect a mobile device from the Internet and want to reconnect through a different subnet, you have to configure the device with a new IP address, and the appropriate netmask and default router. Otherwise, routing protocols have no means of delivering packets because the device's IP address doesn't contain the necessary information about the current point of attachment to the Internet.
All the variations of Mobile IP assign each mobile node a permanent home address on its home network and a care-of address that identifies the current location of the device within a network and its subnets. Each time a user moves the device to a different network, it acquires a new care-of address. A mobility agent on the home network associates each permanent address with its care-of address. The mobile node sends the home agent a binding update each time it changes its care-of address using Internet Control Message Protocol (ICMP). In Mobile IPv4, traffic for the mobile node is sent to the home network but is intercepted by the home agent and forwarded via tunneling mechanisms to the appropriate care-of address. Foreign agents on the visited network help to forward datagrams. Mobile IPv6 was developed to minimize the necessity for tunneling and to include mechanisms that make foreign agents unnecessary.
Enhancements to the Mobile IP standard, such as Mobile IPv6 and Hierarchical Mobile IPv6 (HMIPv6), were developed to advance mobile communications by making the processes involved less cumbersome. Although the North American mobile trend is not moving as quickly as some other markets, the growing adoption of mobile communications elsewhere is likely to drive acceptance globally. According to a Gartner Group report, by 2004 40% of all business-to-business (B2B) transactions outside of North America will be initiated by mobile devices.