Public network IPv4 address of NAT device and private IPv4 address of host at client end under NAT device are packed at low 64 bits of IPv6 address. Further, using prefix published by borderline router constructs IPv6 address, which is assigned to the said host at client end. When host at client end sends message to IPv6 network or receives message from IPv6 network, NAT device and borderline router carry out second time of tunnel pack and de-pack so as to form tunnel relay. Thus, message of overcrossing NAT device can be transferred between private network and IPv6 network. Ensuring that the tunnel crosses over NAT device, the invention simplifies network structure, lowers deployment difficulty and raises communication efficiency. |