An Internet Protocol (IP) address allocation method and a device to resolve a technical problem that an IP address conflict occurs when a control plane device and a user plane device allocate an IP address for one user equipment (UE) at the same time. When receiving a session establishment request message, a control plane network device may determine whether a first user plane network device is capable of allocating an IP address for a terminal device. If the first user plane network device is capable of allocating the IP address for the terminal device, the control plane network device may send an address allocation indication to the first user plane network device.