A data processing apparatus comprising: first control means arranged to control the data processing apparatus; registration means arranged to register definition information including identification information and process information, the identification information identifying a type of a packet to be received from an external apparatus via a network, the process information indicating a process to be performed on the packet; electric power control means arranged to control supplying of electric power such that electric power is supplied to the first control means in a normal power mode and supplying of electric power is stopped at least to the first control means in a power saving mode; and second control means arranged to, in response to a packet corresponding to the type being received from an external apparatus while the data processing apparatus operates in the power saving mode, identify definition information corresponding to the type of the received packet based on the identification information included in the definition information registered in the registration means, and then to perform a process according to the process information included in the identified definition information without shifting a power mode of the data processing apparatus from the power saving mode to the normal power mode, characterized in that the second control means is further configured to control registration such that definition information including identification information and process information is updated in the registration means according to an instruction received from the external apparatus, the identification information identifying a type of a packet to be received from the external apparatus via a network, the process information indicating a process to be performed on the packet. first control means arranged to control the data processing apparatus; registration means arranged to register definition information including identification information and process information, the identification information identifying a type of a packet to be received from an external apparatus via a network, the process information indicating a process to be performed on the packet; electric power control means arranged to control supplying of electric power such that electric power is supplied to the first control means in a normal power mode and supplying of electric power is stopped at least to the first control means in a power saving mode; and second control means arranged to, in response to a packet corresponding to the type being received from an external apparatus while the data processing apparatus operates in the power saving mode, identify definition information corresponding to the type of the received packet based on the identification information included in the definition information registered in the registration means, and then to perform a process according to the process information included in the identified definition information without shifting a power mode of the data processing apparatus from the power saving mode to the normal power mode, characterized in that the second control means is further configured to control registration such that definition information including identification information and process information is updated in the registration means according to an instruction received from the external apparatus, the identification information identifying a type of a packet to be received from the external apparatus via a network, the process information indicating a process to be performed on the packet.