A method for forwarding data packet, a communication system, a Carrier Grade Network Address Translation (CGN) device, and an access gateway are provided. The CGN device communicates with the access gateway through an Internet Protocol Version 4 (IPv4) network and is used for receiving a first data packet forwarded by the access gateway and determining whether the first data packet is an IPv4 packet or an IPv6-over-IPv4 tunnel packet; forwarding the first data packet to the IPv4 network if the first data packet is the IPv4 packet; and decapsulating the first data packet if the first data packet is the IPv6-over-IPv4 tunnel packet to a first IPv6 packet, and sending the first IPv6 packet to an IPv6 network or the IPv4 network. Therefore, technical problems of fully utilizing the IPv4 network and, gradually increasing deployment of the IPv6 network, and realizing communication are solved.