The Intranet, like the Internet, is used to share and access information. However, unlike the Internet, the sharing and accessing of information on an intranet is limited to a private network. You can consider the intranet as the Internet within an organization. The intranet is more secure

as compared to the Internet because the information is shared within an organization. The internal network of a commercial bank is an example of an internet. All the computers in a bank are interconnected. This enables real-time sharing and updating of information, such as account balance.

The Extranet is a network that allows authorized users outside an organization to access and share information. Users can include clients, business partners, and vendors taking resource to electronic transactions. An Automatic Teller machine (ATM) is an example of an extranet. ATMs allow account holders to check the status of their bank accounts and deposit and withdraw money.


Proxy Servers
Proxi servers are software programs that act as an intermediary link between the client, such as a Web browser, and the Web server that stores the information that the client requests for. Proxy servers enable sharing of the same Internet connection among multiple Clients.

FireWalls
A firewall is a security mechanism to protect the data in a networked computer from the other networks with which it interacts, such as the internet.

A firewall can be:

• Hardware-based
• Software-based

In addition, a firewall can be a combination of both the above-mentioned types. Regardless of the way in which it is implemented, a firewall prevents the computers within a privat network form being accessed directly by a public network or other external or unauthorized users. All communication takes place through the firewall, which examines all the incoming data before allowing it to enter the network, as shown in the following figure:

Picture [Implementation of Firewall in a Network]

As depicted in the preceding figure, a request to access a network resource passes through the firewall. The firewall authenticates the user who makes the request and allows or denies the reuquest.

Viruses
A virus is a destructive computer program that infects computers by replicating itself and spreading across the network. Viruses can easly hide within a computer by attaching themselves to other files or programs because they are small. Viruses normally either destory data or affect the function of the software applications on a computer.