A data transmission method includes: receiving, by the PNF SDN controller, a first packet and an access loop identifier that are sent by a first network device; generating, by the PNF SDN controller, a second packet based on the access loop identifier and the first packet; and sending, by the PNF SDN controller, the second packet to the virtualized network function software-defined networking VNF SDN controller, so that the VNF SDN controller sends the second packet to a second network device, where the second packet is used to request the second network device to assign an IP address or IP address prefix to user equipment.