A method for allocating an address in an IP network includes: receiving, by a server, a request message from a client, where the client is connected to the server through at least one routing device, and the request message received by the server carries a device identifier of a preceding stage routing device of the client; obtaining, by the server, according to the device identifier of the preceding stage routing device, a first address prefix pool that is pre-allocated to the preceding stage routing device, and allocating a second address prefix pool to the client according to the first address prefix pool, where an address range corresponding to the second address prefix pool falls within an address range corresponding to the first address prefix pool; and sending, by the server, the second address prefix pool to the client.