What is the difference between internal and external modem

A modem converts digital data from a computer into signals that can travel over a communication line, and back again, allowing a computer to connect to a network or the internet. The distinction between the two main types of modem lies in how each one is physically connected to the computer. One type is built as an expansion card, following either of two internal card standards, and is fitted directly inside the computer's case, plugging into a slot on the motherboard so that it draws power and exchanges data internally. The other type is a separate, standalone unit that sits outside the computer case and is traditionally connected to the machine through an external serial port, requiring its own power supply and a cable running to the computer.

The reverse pairing, describing the expansion-card modem as external and the serial-port modem as internal, gets the two types backwards and does not match how each is actually connected. Describing one type as always being a USB device confuses a specific, more modern connection method with the general external type, since traditional external modems used serial ports rather than USB. Stating that both types are expansion cards is also incorrect, since only the internal type plugs directly into the motherboard as a card; the external type is a separate unit rather than a card at all.

The clearest way to remember this distinction is that "internal" modems live inside the computer as expansion cards, while "external" modems sit outside the case and connect through a port.