The present invention provides network-layer authentication protocols for authenticating mobile client and access router to each other. The present invention uses Router Discovery as a carrier to implement the authentication protocols. In an embodiment of the present invention, a mobile client sends out a solicitation message to request connectivity service. The solicitation message contains a proof of identity of the mobile client. An access router that receives the solicitation message will not respond to it until the proof of the identity is verified. Only when the proof of identity of the mobile client is verified, will the access router respond and return an advertising message to the mobile client, thereby preventing unauthorized mobile clients from obtaining network access.