Routers are the traffic cops of intranets. They make sure that all data gets sent to where it's supposed to go and that it gets sent via the most efficient route. Routers are also useful tools to make the most efficient use of the intranet. Routers are used to segment traffic and provide redundancy of routes. Routers use encapsulation to permit different protocols to be sent across otherwise incompatible networks.
When you sit down at your computer on an intranet and send or receive data, that information generally must first go through at least one router, and often more than one router before it reaches its final destination. Routers can be simple or quite sophisticated. Factors that determine the required complexity of a router include the size of the intranet, the type and quantity of traffic on segments, and security concerns of the intranet. The more complex the intranet, and, in particular, the greater number of possible destinations for data, the greater the need for sophisticated router hardware and software.
Routers open the IP packet to read the destination address, calculate the best route, and then send the packet toward the final destination. If the destination is on the same part of an intranet, the packet would be sent directly to the destination computer by the router. If the packet is destined for another intranet or subnetwork (or if the destination is on the Internet), the router considers factors like traffic congestion and the number of hops-a term that refers to the number of routers or gateways on any given path. The IP packet carries with it a segment that holds the hop count and a router will not use a path that would exceed a predefined number of hops. Multiple routes within an acceptable hop count range are desirable in intranets to provide redundancy and assure that data can get through. For example, if a direct route between San Francisco and New York were unavailable, sophisticated routers would send data to New York via another router probably in another city on the intranet-and this would all be transparent to the users.
Routers have two or more physical ports: receiving (input) ports and sending (output) ports. In actuality, every port is bi-directional and can receive or send data. When a packet is received at an input port, a software routine called a routing process is run. This process looks inside the header information in the IP packet and finds the address where the data is being sent. It then compares this address against an internal database called a routing table