A method for message transmission, the method comprising: according to a received request message sent from an internal network device, assigning, by a network address translation, NAT, device, a range of external ports sent from the internal network device to the NAT device and a range of external ports sent from the NAT device to an external network, the range of external ports sent from the internal network device to the NAT device is the same as the range of external ports sent from the NAT device to the external network, and transmitting, by the NAT device, to the internal network device a response message including the range of external ports, wherein the request message comprises a Pinhole Control Protocol, PCP, message or a Dynamic Host Configuration Protocol, DHCP, message; establishing, by the NAT device, the NAT entries according to the range of external ports; receiving, by the internal network device, a forward message sent from a user equipment to the external network; transmitting, by the internal network device, the forward message after replacing a source port number of the forward message with the range of external ports, and replacing a source address thereof with a private network IP address of an internal network device; receiving, by the NAT device, the forward message sent from the internal network device to the external network; and transmitting, by the NAT device, the forward message after replacing a source address of the forward message with a public network IP address according to a private network IP address, a range of external ports and the public network IP address of the internal network device, which are contained in network address translation NAT entries; wherein alternative to receiving, by the NAT device, the forward message sent from the internal network device to the external network; and transmitting, by the NAT device, the forward message after replacing a source address of the forward message with a public network IP address according to a private network IP address, a range of external ports and the public network IP address of the internal network device, which are contained in network address translation NAT entries; the method comprises: receiving, by the NAT device, a reverse message sent from the external network to the internal network device; and transmitting, by the NAT device, the reverse message after replacing a destination public network IP address of the reverse message sent from the external network to the internal network device, with the private network IP address of the internal network device according to the private network IP address, the range of external ports and the public network IP address of the internal network device, which are contained in the network address translation, NAT, entries. according to a received request message sent from an internal network device, assigning, by a network address translation, NAT, device, a range of external ports sent from the internal network device to the NAT device and a range of external ports sent from the NAT device to an external network, the range of external ports sent from the internal network device to the NAT device is the same as the range of external ports sent from the NAT device to the external network, and transmitting, by the NAT device, to the internal network device a response message including the range of external ports, wherein the request message comprises a Pinhole Control Protocol, PCP, message or a Dynamic Host Configuration Protocol, DHCP, message; establishing, by the NAT device, the NAT entries according to the range of external ports; receiving, by the internal network device, a forward message sent from a user equipment to the external network; transmitting, by the internal network device, the forward message after replacing a source port number of the forward message with the range of external ports, and replacing a source address thereof with a private network IP address of an internal network device; receiving, by the NAT device, the forward message sent from the internal network device to the external network; and transmitting, by the NAT device, the forward message after replacing a source address of the forward message with a public network IP address according to a private network IP address, a range of external ports and the public network IP address of the internal network device, which are contained in network address translation NAT entries; receiving, by the NAT device, a reverse message sent from the external network to the internal network device; and transmitting, by the NAT device, the reverse message after replacing a destination public network IP address of the reverse message sent from the external network to the internal network device, with the private network IP address of the internal network device according to the private network IP address, the range of external ports and the public network IP address of the internal network device, which are contained in the network address translation, NAT, entries.