Adapted from: http://en.wikipedia.org/wiki/Routing

Routing directs forwarding, the passing of logically addressed packets from their source toward their ultimate destination through intermediary nodes (called routers). The routing process usually directs forwarding on the basis of routing tables within the routers, which maintain a record of the best routes to various network destinations. Thus the construction of routing tables becomes very important for efficient routing.

Routing differs from bridging in its assumption that address-structures imply the proximity of similar addresses within the network, thus allowing a single routing-table entry to represent the route to a group of addresses. Therefore, routing outperforms bridging in large networks, and it has become the dominant form of path-discovery on the Internet.

Small networks may involve hand-configured routing tables. Large networks involve complex topologies and may change constantly, making the manual construction of routing tables very problematic. Nevertheless, most of the public switched telephone network (PSTN) uses pre-computed routing tables, with fallback routes if the most direct route becomes blocked; see routing in the PSTN. Dynamic routing attempts to solve this problem by constructing routing tables automatically, based on information carried by routing protocols, and allowing the network to act nearly autonomously in avoiding network failures and blockages.

Dynamic routing dominates the Internet. However, the configuration of the routing protocols often requires a skilled touch; one should not suppose that networking technology has developed to the point of the complete automation of routing.

Packet-switched networks, such as the Internet, split data up into packets, each labeled with the complete destination address and each routed individually. Circuit switched networks, such as the voice telephone network, also perform routing, in order to find paths for circuits (such as telephone calls) over which they can send large amounts of data without continually repeating the complete destination address.

The hardware used in routing includes hubs, switches, and routers.

Ethernet Hub:

An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fibre optic Ethernet devices together, making them act as a single segment. It works at the physical layer of the OSI model, repeating the signal received at one port out each of the other ports (but not the original one). The device is thus a form of multiport repeater. Ethernet hubs are also responsible for forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or AUI connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced Ethernet switches has largely rendered hubs obsolete but they are still seen in older installations and more specialist applications.

Network Switch:

A network switch (or just switch for short) is a networking device that performs transparent bridging (connection of multiple network segments with forwarding based on MAC addresses) at full wire speed in hardware. The use of specially designed hardware also makes it possible to have large numbers of ports (unlike a PC based bridge which is very limited by expansion slot count).

If a network has only switches and no hubs then the collision domains are either reduced to a single link or, if both ends support full duplex, eliminated altogether. The principle of a fast hardware forwarding device with many ports can be extended to higher layers giving the multilayer switch.

Router:

A router is a computer networking device that forwards data packets across an internetwork toward their destinations, through a process known as routing. Routing occurs at layer 3 (the Network layer e.g. IP) of the OSI seven-layer protocol stack.