A mobile node uses a registration request message to register with the home agent. Thus, the home agent can create or modify a mobility binding for that mobile node (for example, with a new lifetime). The foreign agent can relay the registration request to the home agent. However, if the mobile node is registering a co-located care-of address, then the mobile node can send the registration request directly to the home agent. If the foreign agent advertises that registration messages must be sent to the foreign agent, then the mobile node must send the registration request to the foreign agent.