Mobile IP (or IP mobility) 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 a permanent IP address. Mobile IP for IPv4 is described in IETF RFC 5944, and extensions are defined in IETF RFC 4721. Mobile IPv6, the IP mobility implementation for the next generation of the Internet Protocol, IPv6, is described in RFC 6275.
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The Mobile IP protocol allows location-independent routing of IP datagrams on the Internet. Each mobile node is identified by its home address disregarding its current location in the Internet. While away from its home network, a mobile node is associated with a care-of address which identifies its current location and its home address is associated with the local endpoint of a tunnel to its home agent. Mobile IP specifies how a mobile node registers with its home agent and how the home agent routes datagrams to the mobile node through the tunnel.
Mobile IP is most often found in wired and wireless environments where users need to carry their mobile devices across multiple LAN subnets. Examples of use are in roaming between overlapping wireless systems, e.g., IP over DVB, WLAN, WiMAX and BWA.
Mobile IP is not required within cellular systems such as 3G, to provide transparency when Internet users migrate between cellular towers, since these systems provide their own data link layer handover and roaming mechanisms. However, it is often used in 3G systems to allow seamless IP mobility between different packet data serving node (PDSN) domains.
Operational principles 
A mobile node has two addresses - a permanent home address and a care-of address (CoA), which is associated with the network the mobile node is visiting. Two kinds of entities comprise a Mobile IP implementation:
- A home agent stores information about mobile nodes whose permanent home address is in the home agent's network.
- A foreign agent stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses, which are used by Mobile IP. If there is no foreign agent in the host network, the mobile device has to take care of getting an address and advertising that address by its own means.
A node wanting to communicate with the mobile node uses the permanent home address of the mobile node as the destination address to send packets to. Because the home address logically belongs to the network associated with the home agent, normal IP routing mechanisms forward these packets to the home agent. Instead of forwarding these packets to a destination that is physically in the same network as the home agent, the home agent redirects these packets towards the remote address through an IP tunnel by encapsulating the datagram with a new IP header using the care of address of the mobile node.
When acting as transmitter, a mobile node sends packets directly to the other communicating node, without sending the packets through the home agent, using its permanent home address as the source address for the IP packets. This is known as triangular routing or "route optimization" (RO) mode. If needed, the foreign agent could employ reverse tunneling by tunneling the mobile node's packets to the home agent, which in turn forwards them to the communicating node. This is needed in networks whose gateway routers check that the source IP address of the mobile host belongs to their subnet or discard the packet otherwise. In Mobile IPv6 (MIPv6), "reverse tunneling" is the default behaviour, with RO being an optional behaviour.
A performance evaluation of Mobile IPv6, carried out by NEC Europe, can be found at the ACM Digital Library, under the entry "A simulation study on the performance of mobile IPv6 in a WLAN-based cellular network", from the Elsevier Computer Networks Journal (CNJ), special issue on The New Internet Architecture, September 2002.
Additionally, a performance comparison between Mobile IPv6 and some of its proposed enhancements (Hierarchical Mobile IPv6, Fast Handovers for Mobile IPv6 and their Combination) is available under the entry "A performance comparison of Mobile IPv6, Hierarchical Mobile IPv6, fast handovers for Mobile IPv6 and their combination", from the ACM SIGMOBILE Mobile Computing and Communications Review (MC2R), Volume 7, Issue 4, October, 2003.
Enhancements to the Mobile IP technique, such as Mobile IPv6 and Hierarchical Mobile IPv6 (HMIPv6) defined in RFC 5380, are being developed to improve mobile communications in certain circumstances by making the processes more secure and more efficient. HMIPv6 explanation can be found at Hierarchical-Mobile-IPv6.
Researchers create support for mobile networking without requiring any pre-deployed infrastructure as it currently is required by MIP. One such example is Interactive Protocol for Mobile Networking (IPMN) which promises supporting mobility on a regular IP network just from the network edges by intelligent signalling between IP at end-points and application layer module with improved quality of service.
Researchers are also working to create support for mobile networking between entire subnets with support from Mobile IPv6. One such example is Network Mobility (NEMO) Network Mobility Basic Support Protocol by the IETF Network Mobility Working Group which supports mobility for entire Mobile Networks that move and to attach to different points in the Internet. The protocol is an extension of Mobile IPv6 and allows session continuity for every node in the Mobile Network as the network moves.
Changes in IPv6 for Mobile IPv6 
- A set of mobility options to include in mobility messages
- A new Home Address option for the Destination Options header
- A new Type 2 Routing header
- New Internet Control Message Protocol for IPv6 (ICMPv6) messages to discover the set of home agents and to obtain the prefix of the home link
- Changes to router discovery messages and options and additional Neighbor Discovery options
- Foreign Agents are no longer needed
Definition of terms 
- Home network
- The home network of a mobile device is the network within which the device receives its identifying IP address (home address).
- Home address
- The home address of a mobile device is the IP address assigned to the device within its home network.
- Foreign network
- A foreign network is the network in which a mobile node is operating when away from its home network.
- Care-of address
- The care-of address of a mobile device is the network-native IP address of the device when operating in a foreign network.
- Home agent
- A home agent is a router on a mobile node’s home network which tunnels datagrams for delivery to the mobile node when it is away from home. It maintains current location (IP address) information for the mobile node. It is used with one or more foreign agents.
- Foreign agent
- A foreign agent is a router that stores information about mobile nodes visiting its network. Foreign agents also advertise care-of-addresses which are used by Mobile IP.
- A binding is the association of the home address with a care-of address...
See also 
- Mobility management
- GPRS Tunneling Protocol
- Vertical handoff
- Proxy Mobile IP
- Host Identity Protocol
- Locator/Identifier Separation Protocol
- X.Pérez-Costa and H.Hartenstein. A Simulation Study on the Performance of Mobile IPv6 in a WLAN-Based Cellular Network Elsevier Computer Networks Journal, special issue on The New Internet Architecture, September 2002
- X.Pérez-Costa, M.Torrent-Moreno and H.Hartenstein. A Simulation Study on the Performance of Hierarchical Mobile IPv6 In Proceedings of the International Teletraffic Congress (ITC), Berlin, Germany, August 2003.
- RFC 6275 -Mobility support for IPv6
- RFC 5944 - IP Mobility Support for IPv4, Revised
- RFC 4721 - Mobile IPv4 Challenge/Response Extensions
- RFC 3024 - Reverse Tunneling for Mobile IP
- Inside Mobile IP
- Protocols for Adaptive Mobile and Wireless Networking
- Mobile IP explained (a tutorial)
- Mobility Extensions for IPv6 (mext) IETF Working Group Web site
- Mobile IPv6 -- A short introduction by Holger Zuleger
- Linux Mobile IPv6 HOWTO on the Linux Documentation Project
- D. Johnson, C. Perkins, J. Arkko. Mobility Support in IPv6. RFC 6275. June 2011
- J. Arkko, V. Devarapalli, F. Dupont. Using IPsec to Protect Mobile IPv6 Signaling Between Mobile Nodes and Home Agents. RFC 3776. June 2004