The backbone network connects the INs and the Internet gateway together. It is this backbone network that helps us provide mobility to all MNs across INs. When an MN moves from one IN (Home Agent) to another IN (Foreign Agent), the foreign agent needs to inform the home agent about the new location of the MN. Moreover all packets to the MN, which will reach the home agent2.2, are tunneled to the foreign agent. This IN-IN communication takes place on the backbone network.
All packets destined to the external world (Internet) are forwarded by the IN to the Internet gateway, which is reachable on the backbone network.
Every IN manages its own subnet. Each of these INs and other routers in the backbone network should know the addresses of all INs and the subnets they manage. This is required, as packets destined to a particular MN have to pass through its associated IN. Also, when an IN wants to route a packet to an MN not in its subnet, the packet has to be forwarded to the appropriate IN.