Disclosed is a method for a device without unique hardware address to obtain IP address automatically. This method comprises the steps of: the device dividing the hardware address field in the BOOTP request packet into sub-fields which are filled in by the device and the relay proxy servers forwarding the BOOTP request packet; the BOOTP server assigning an IP address to the device according to the contents in all sub-fields in the received BOOTP request packet, then generating a BOOTP response packet including the IP address of the device; and the device obtaining the IP address information from the BOOTP response packet forwarded by relay proxy servers. The content of the sub-fields can be information of interface board number, port number, slot number or Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI) of an ATM Permanent Virtual Connection (PVC) of said device. Each sub-field alone cannot be a unique identifier of the device, but combination of all sub-fields can be used for identifying the device uniquely. With this method, a device can automatically obtain an IP address through BOOTP, so configuration and maintenance for devices becomes simple.