what is router?

A router is a device that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway, including each point-of-presence on the Internet. A router is often included as part of a network switch.

Routers may be programmed to filter out some packets, and to dynamically change the route by which packets are routed. Routers often use different media on each interface. For instance, a router might have one Ethernet port and one ISDN port.

The function of a router is to direct data along the most efficient and economical route to the destination device. Routers operate at Network Layer 3, which means they examine the logical network address (for example, 191.29.21.100) and not the physical hardware address (MAC).

Routers are smarter than bridges because they know about routing protocols, different address schemes, different frame sizes and different data rates in order to make the best decision on which path to choose. The best path is determined by using routing tables and algorithms.

Routers were devised in order to separate networks logically. For instance, a TCP/IP router can segment the network based on IP subnets. Most
routers can also perform bridging functions. A major feature of routers, because they can filter packets at a protocol level, is to act as a firewall. This is essentially a barrier, which prevents unwanted packets either entering or leaving the network.

Bridging and routing are both ways of performing data control, but work through different methods. Bridging takes place at OSI Model Layer 2 (Data-Link Layer) while Routing takes place at the OSI Model Layer 3 (Network Layer). This difference means that a bridge directs frames according to hardware assigned MAC addresses while a router makes its decisions according to arbitrarily assigned IP Addresses. As a result of this, bridges are not concerned with and are unable to distinguish networks while routers can.