IPv4 addresses are 32 bits long, but IPv6 interfaces are identified by 128-bit addresses. The socket interface makes the size of an IP address visible to an application; virtually all TCP/IP applications using sockets have knowledge of the size of an IP address. Those parts of the API that expose the addresses must be changed to accommodate the larger IPv6 address size. IPv6 also introduces new features, some of which must be made visible to applications by way of the API.