Hub-Switch-Router-Proxy-Firewall ?

Definition of Hub, Switch , Router, Proxy and Firewall ?
Hubs
The term hub is sometimes used to refer to any piece of network equipment that connects PCs together, but it actually refers to a multi-port repeater. This type of device simply passes on (repeats) all the information it receives, so that all devices connected to its ports receive that information.

Hubs repeat everything they receive and can be used to extend the network. However, this can result in a lot of unnecessary traffic being sent to all devices on the network. Hubs pass on traffic to the network regardless of the intended destination; the PCs to which the packets are sent use the address information in each packet to work out which packets are meant for them. In a small network repeating is not a problem but for a larger, more heavily used network, another piece of networking equipment (such as a switch) may be required to help reduce the amount of unnecessary traffic being generated.

Switches

Switches control the flow of network traffic based on the address information in each packet. A switch learns which devices are connected to its ports (by monitoring the packets it receives), and then forwards on packets to the appropriate port only. This allows simultaneous communication across the switch, improving bandwidth This switching operation reduces the amount of unnecessary traffic that would have occurred if the same information had been sent from every port (as with a hub).

Router

A device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP network. Routers are located at gateways, the places where two or more networks connect.
A router is placed at a junction between two or more networks/segments; each network/segment can be using a different topology or protocol.

Proxy

A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server.

Proxy servers are specialized web servers that allow web browsers to receive web pages from web servers without communicating with them directly. Proxy servers are often used to provide more secure web access in organizations; the proxy server is allowed to connect to the Internet, but the individual web browsers are only allowed to "talk" to the proxy server.

When there are many users sharing a single proxy server, the proxy server can also speed up web browsing by caching popular pages.

Firewall

A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

A firewall is a system that is set up to control traffic flow between two networks. Firewalls are most commonly specially configured UNIX systems, but firewalls have also been built out of many other systems, including systems designed specifically for use as firewalls.

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