A computer includes an enclosure, a network card connected to an external network, a time limit control device, a power module, and a switch. The time limit control device is used to set a connection time of the computer with the external network, and control a network switch module to enable the network card to connect the computer to the external network according the set connection time. The time limit control device is used to count down the connection time, and control a display module to display a remaining connection time. When a countdown of the connection time is zero, the time limit control device controls the network switch module to disable the network card and thereby disconnecting the computer from the external network.