What is Mobile IP?

A standard that allows users with mobile devices whose IP addresses are associated with one network to stay connected when moving to a network with a different IP address.

When a user leaves the network with which his device is associated (home network) and enters the domain of a foreign network, the foreign network uses the Mobile IP protocol to inform the home network of a care-of address to which all packets for the user’s device should be sent.

Mobile IP is most often found in wireless WAN environments where users need to carry their mobile devices across multiple LANs with different IP addresses.

A common analogy to explain Mobile IP is when someone moves his residence from one location to another. Person moves from Boston to New York. Person drops off new mailing address to New York post office. New York post office notifies Boston post office of new mailing address. When Boston post office receives mail for person it knows to forward mail to person’s New York address.


Mobile IP is most useful in environments where mobility is desired and the traditional land line dial-in model or DHCP do not provide adequate solutions for the needs of the users. If it is necessary or desirable for a user to maintain a single address while they transition between networks and network media, Mobile IP can provide them with this ability. Generally, Mobile IP is most useful in environments where a wireless technology is being utilized. This includes cellular environments as well as wireless LAN situations that may require roaming. Mobile IP can go hand in hand with many different cellular technologies like CDMA, TDMA, GSM, AMPS, NAMPS, as well as other proprietary solutions, to provide a mobile system which will scale for many users.

Each mobile node is always identified by its home address, no matter what its current point of attachment to the Internet, allowing for transparent mobility with respect to the network and all other devices. The only devices which need to be aware of the movement of this node are the mobile device and a router serving the user’s topologically correct subnet.

List of Terms

  • agent discovery –  The method by which a mobile node determines whether it is currently connected to its home network or a foreign network and detects whether it has moved and the way it has moved. It is the mechanism by which mobile nodes query and discover mobility agents. This is done is through an extension of the ICMP router discovery protocol, IRDP (RFC 1256),which includes a mechanism to advertise mobility services to potential users.
  • care-of address –  The termination point of the tunnel to a mobile node. This can be a collocated care-of address, where the mobile node acquires a local address and detunnels its own packets, or a foreign agent care-of address, where a foreign agent detunnels packets and forwards them to the mobile node.
  • correspondent node –  A peer with which a mobile node is communicating. A correspondent node may be either stationary or mobile.
  • foreign agent –  A router on a mobile node’s visited network which provides routing services to the mobile node while registered. The foreign agent detunnels and delivers datagrams to the mobile node that were tunneled by the mobile node’s home agent. For datagrams sent by a mobile node, the foreign agent may serve as a default router for registered mobile nodes.
  • home address –  An IP address that is assigned for an extended time to a mobile node. It remains unchanged regardless of where the node is attached to the Internet.
  • home agent –  A router on a mobile node’s home network which tunnels packets to the mobile node while it is away from home. It keeps current location information for registered mobile nodes called a mobility binding.
  • home network – The network or virtual network which matches the subnet address of the mobile node.
  • mobile node –  A host or router that changes its point of attachment from one network or subnet to another. A mobile node may change its location without changing its IP address; it may continue to communicate with other Internet nodes at any location using its home IP address, assuming link-layer connectivity to a point of attachment is available.
  • mobility agent –  A home agent or a foreign agent.
  • mobility binding –  The association of a home address with a care-of address and the remaining lifetime.
  • mobility security association –  A collection of security contexts between a pair of nodes, which may be applied to Mobile IP protocol messages exchanged between them. Each context indicates an authentication algorithm and mode, a secret (a shared key or appropriate public/private key pair), and a style of replay protection in use.
  • MTU –  Maximum transmission unit. Maximum packet size, in bytes, that a particular interface can handle.
  • node –  A host or router.
  • registration –  The process by which the mobile node is associated with a care-of address on the home agent while it is away from home. This may happen directly from the mobile node to the home agent or through a foreign agent.
  • security parameter index (SPI) –  The index identifying a security context between a pair of nodes.
  • tunnel – The path followed by a datagram while it is encapsulated from the home agent to the mobile node.
  • virtual network –  A network with no physical instantiation beyond a router (with a physical network interface on another network). The router (a home agent, for example) generally advertises reachability to the virtual network using conventional routing protocols.
  • visited network –  A network other than a mobile node’s home network, to which the mobile nodeis currently connected.
  • visitor list – The list of mobile nodes visiting a foreign agent.


[1] Anonym. Mobile IP. www.whatis.com. Accessed November 2010.

[2] Anonym. Mobile IP. Webopedia Computer Dictionary. Accessed November 2010.