Peripheral devices are pieces of hardware that are connected to a computer but external to its central processing unit (CPU). These devices are not part of the essential elements of a computer, like its memory or microprocessor. Some examples include mice, printers, and keyboards, which all attach to computers but do not impact its basic functions. External peripherals connect and operate outside of the unit. Internal peripherals are built into the unit itself. Only the most crucial parts of a computer aren’t considered peripheral; even the monitor is part of the peripheral class of devices. This subset of hardware is also sometimes referred to as I/O devices because they provide input, output, or both to the computer.
In system access control, all of the hardware that makes up your network is considered external — it all connects to a central computer, but none of it is actually controlled by the computer’s CPU. They work in tandem, but are still distinct devices.