In a portal authentication method, a DHCP request message sent by a terminal is received by an AC. In response to finding that a user of the terminal is an unauthenticated user, a private network IP address is assigned to the terminal. After portal authentication of the terminal is finished, a wireless connection of the terminal is terminated by the AC. When a DHCP request message sent by the terminal again is received, a determination that the user of the terminal passes the authentication is made by the AC, a public network IP address is assigned to the terminal, and an accounting request message is sent to a RADIUS server. After finding that the terminal is offline, an accounting stop message is sent by the AC to the RADIUS server, the wireless connection of the terminal is disconnected, and the public network IP address is released.