A router is a computer network device that forwards data packets between computer networks. It is connected to two or more data lines from different networks and decides which way to send each data packet based on its current understanding of the state of the networks it is connected to. Routers are used to connect networks of different sizes, such as local area networks (LANs) and wide area networks (WANs). They can also be used to connect two or more logical subnets, which are networks that use the same network address range. Routers use routing tables to determine where to send data packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts.