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In a telecommunications network, a switch is a device that channels incoming data from any of the multiple input ports to the specific output port that will carry the data to its intended destination. In the traditional circuit-switched telephone network, one or more switches are used to establish a dedicated but temporary connection or circuit for an exchange between two or more parties. In an Ethernet local area network (LAN), a switch determines from the address of the physical device (Media Access Control or MAC) in each incoming message frame the output port to forward it to and from. In a large packet switching network, such as the Internet, a switch determines from the IP address in each packet the output port to be used for the next part of its trip to the intended destination.

In the Open Systems Interconnect (OSI) communications model, a switch performs layer 2 or data link layer function. That is, it simply looks at each packet or unit of data and determines from a physical address (the "MAC address") to which device a data unit is destined and switches it to that device. However, in wide area networks such as the Internet, the destination address requires a search in a routing table for a device known as a router. Some newer switches also perform routing functions (layer 3 or network layer functions in OSI) and are sometimes called IP switches.

In larger networks, the journey from one switching point to another on the network is called a jump. The time it takes for a switch to figure out where to forward a data unit is called latency. The price paid for having the flexibility provided by switches in a network is this latency. Switches are at the levels of the backbone and the gateway of a network in which one network connects with another and at the subnet level where the data is forwarded near its destination or source. The former are often referred to as kernel switches and the latter as desktop switches.