All the variations of Mobile IP assign each mobile node a permanent home address on its home network and a care-of address that identifies the current location of the device within a network and its subnets.
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. Now, a tunnel will be established between the home agent and the foreign agent by the process of tunneling.
The Foreign Agent is a router that advertises care-of addresses, which are used by mobile IP.
The address might be dynamically acquired as a temporary address by the mobile node, such as through DHCP. In the case of the mobile node, this address is its home address. For each registered mobile node, the home agent forwards received broadcast datagrams to the mobile node; the method depends on how the configuration of the home agent specifies categories of broadcast datagrams forwarded to mobile nodes.
If the mobile node's home agent is a multicast router, the mobile node can join groups using a bidirectional tunnel to its home agent. If the registration request is not valid, the Foreign Agent sends a registration reply with appropriate error code to the Mobile Node.
This is known as agent solicitation. The foreign agent forwards the datagram to the Internet host.
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