A port expander is a device that allows one port on a computer system to connect to multiple devices. There are two basic forms of port expander: internal and external. An internal expander has a connection inside the computer, typically on the motherboard, and the only part the user sees is the expansion plate containing multiple ports. An external device plugs into the existing port and then has multiple places to connect. When not part of a computer system, these devices are commonly known as splitters.
In the non-computer world, splitters are very common. Extension cords and power strips are in nearly every modern home. Both of these devices will split a single outlet to multiple devices. Cable splitters are also in many homes, allowing a single coaxial cable to provide cable television to multiple sets. Some systems may even use an A/B box, a device that connects multiple sets of devices to the same system, but switches between them by flipping between the A or B mode.
These devices are all doing the same basic job that a port expander does. The expander will connect to a single spot, but have multiple connections for devices. They go by several names, such as switch, hub or splitter, but they all do the same thing. Expanders are made for nearly every type of port, but the most common household versions are for universal serial bus (USB) and Ethernet ports.
An Internal port expander is common on desktop systems. A single expansion plate will have multiple ports, often several USB. Inside the computer, the ports are all part of a single card that will plug into a predesignated spot on the motherboard. The motherboard spot is programmed to handle a single port, but the card that connects to the external ports handles the routing so the system isn’t overloaded.
An external port expander is usually much more obvious. A cable will run from the computer to a secondary box. That box will have several spots that are identical to the one on the main system. For example, an Ethernet hub connects to the network port on a computer and then splits the connection many different ways. This will allow multiple network devices to share the same port.
On commercial systems, there are a large number of different types of port expander. Servers often have much different power and load requirements than standard user systems. They will commonly use an external device to route and monitor usage though the various ports. These systems range from ones similar to a home system all the way to specialized internal hardware that is built to connect to external expanders.