Mobile IP is a routing protocol that allows hosts (the ''mobile nodes'') to
retain their IP address while changing their point of Internet connectivity. Traffic to a mobile node is forwarded to the mobile node by its
''home agent,''
which uses ProtocolTunneling (usually IPIP) to forward the mobile node's
''foreign agent.''
The foreign agent then detunnels the packet and sends it over the final link to the mobile node.  Note that a mobile node can function as its own foreign agent if it is able to obtain an IP address at its current location (via DHCP, for
example).

If you want to know more, go visit http://www.cs.pdx.edu/research/SMN and tell them BillTrost sent you.