(1) On x86-based PCs, a number used to identify a peripheral device, such as a disk, sound card or serial port. The address, which is expressed as a hex number (1F0, 37F, etc.), must be unique. Until the late 1990s, controller cards typically contained a small range of selectable addresses to avoid conflicts with other devices. However, the Plug and Play feature, introduced with Windows 95, resolved conflicts automatically. See PC I/O addressing.