A printed circuit board that provides a transmitting/receiving port for a particular wide area network (WAN) or local area network (LAN) protocol. Line cards plug into telephone company switches and high-end routers. A line card implies that the unit has a modular chassis that can be expanded by adding more cards and/or that it supports multiple network protocols. Small and medium-size routers have a fixed number of ports, typically Ethernet, and the line card circuitry is built into each port.