Mobile IP

Mobile node
Home Network: a network that the mobile node is permanently registered at and holds an ip address there.
Home address: permanent ip address of the mobile node in the home network
Foreign Network: The network that the mobile node currently resides at

Home Agent: an entity similar to DNS or proxy that registeres where the mobile node is currently located at (mobile node advertises its new foreign network if it changes place)

Node to mobile node: node uses the permanent home address of the mobile node as the destination address to send packets to. normal IP routing mechanisms forward these packets to 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.

Mobile node to node: 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.

Mobile IP Datagram Flow

Triangle Routing
Reference: C.E. Perkins, "Mobile IP" IEEE Communications Magazine, May 1997