Mobile IP

Mobile IP is a protocol for communication between wireless devices.  It was designed by the Internet Engineering Task Force (IETF) to allow wireless users to roam between networks without having to change the IP addresses of mobile devices.  Wireless routers, known as agents, periodically broadcast signals that carry information about the networks to which they are connected.  These messages are called agent advertisements.  When a wireless device, such as a laptop, is powered on for the first time, it scans for an agent advertisement in a process called agent discovery.  Upon discovery of a nearby agent, the wireless device sends out a registration request that contains communication parameters.  If the agent supports communication within the specified parameters, it sends an acknowledgement packet with a permanent IP address to the wireless device and becomes the home agent for the device.  The home agent is responsible for tracking the device as it roams between networks and transmitting messages to it.  This is accomplished through a process known as tunneling, similar to tunneling in virtual private networks.
         The wireless device is allowed to keep the IP address issued by the home agent even if it roams onto another network controlled by another agent.  At this point, the wireless device transmits a solicitation request for temporary association with the foreign agent.  A foreign agent is any agent other than the wireless device’s home agent.  The foreign agent, recognizing that the device is roaming, will issue a temporary IP address to be used as a care-of address for the device to use while it is on the foreign network.  The device then sends its care-of address back to the home agent where it is bound to the device’s permanent IP address.  Any data that is destined for the wireless device from the home network is encapsulated by the home agent with a new header that contains the network portion of the care-of address and sent to the foreign agent.  The foreign agent strips off the new header and sends the data to the mobile device. 
         The current version of mobile IP only supports IPv4 addressing, and is not considered very efficient for enterprise use because messages are required to go through agents rather than directly to the device.  The IETF is currently working on a new version which will support IPv6 addressing, making direct communication between mobile devices possible.