A method of operating an Internet protocol (IP) address that effectively allocates, creates, and processes an interface identifier (ID) of an IP address area, and a subnet system using the same are provided. Allocation is done such that in the IP address area including a subnet prefix area and an Interface ID area, certain bits of the Interface ID area are used as an index area of a subnet gateway. In this case, the allocation is implemented in order from the higher layer to the lower layer so that the Interface ID area is sequentially allocated from the higher bits thereof for the index areas of the respective layers. The combination of the certain bits of the allocated Interface ID and the subnet prefix area of the IP address area is used as the subnet ID of the respective subnet layers. The lowest subnet gateway allocates the Interface ID to terminating equipment trying to access to the IP network based on a format of the IP address.