Link layer authentication information is supplied by a link layer authentication device to an access router for tracking IP address usage by a client device. The authentication information supplied to the access router includes an authenticated client identifier and a corresponding authenticated link identifier for the client device that attached to the network based on the authenticated link identifier. The access router, in response to receiving a message that specifies the authenticated link identifier and a source IP address, adds the source IP address to a cache entry that specifies the authenticated client identifier and the corresponding authenticated link identifier, and outputs to an audit resource a record that specifies the source IP address and the authenticated link identifier.