Routers are devices that connect two or more networks. They consist of a combination of hardware and software. The hardware can be a network server, a separate computer or a special device. The hardware includes the physical interfaces to the various network in the inter network.
These interface can be token ring, Ethernet, Frame relay, ATM or any other technology. The software in a router are the operating system and the routing protocol. Routers use logical and physical addressing to connect to or more logically separated networks. They accomplish this connection by organizing the large network into logical network segments called as sub net. Each sub net has its unique logical address. This allows the networks to be separated, but they can still exchange data.
Data is grouped in packets. Each packet has its unique logical as well as physical address, this allows the router to calculate the optimal path to a computer