A diagnostic method performed by a first communication node among a plurality of communication nodes in a vehicle network may include generating a diagnostic initiation message indicating that a first diagnostic procedure by an external diagnostic apparatus is to be performed upon detecting that the external diagnostic apparatus is connected to the first communication node; transmitting the diagnostic initiation message in a broadcast manner in the vehicle network; receiving a negative response message as a response to the diagnostic initiation message from a second communication node among the plurality of communication nodes, the negative response message indicating the second communication node operating as an internal diagnostic apparatus; and performing the first diagnostic procedure after a time indicated by the negative response message.