Internet
This is the world-wide network of computers accessible to anyone who knows their Internet Protocol (IP) address - the IP address is a unique set of numbers (such as 209.33.27.100) that defines the computer's location. Most will have accessed a computer using a name such as http://tech.QueryHome.com . Before this named computer can be accessed, the name needs to be resolved (translated) into an IP address. To do this your browser (for example Netscape or Internet Explorer) will access a Domain Name Server (DNS) computer to lookup the name and return an IP address - or issue an error message to indicate that the name was not found. Once your browser has the IP address it can access the remote computer. The actual server (the computer that serves up the web pages) does not reside behind a firewall - if it did, it would be an Extranet. It may implement security at a directory level so that access is via a username and password, but otherwise all the information is accessible. To see typical security have a look at a sample secure directory - the username is Dr and the password is Who (both username and password are case sensitive).
Intranet
This is a network that is not available to the world outside of the Intranet. If the Intranet network is connected to the Internet, the Intranet will reside behind a firewall and, if it allows access from the Internet, will be an Extranet. The firewall helps to control access between the Intranet and Internet to permit access to the Intranet only to people who are members of the same company or organisation.
In its simplest form, an Intranet can be set up on a networked PC without any PC on the network having access via the Intranet network to the Internet.
For example, consider an office with a few PCs and a few printers all networked together. The network would not be connected to the outside world. On one of the drives of one of the PCs there would be a directory of web pages that comprise the Intranet. Other PCs on the network could access this Intranet by pointing their browser (Chrome or Internet Explorer) to this directory - for example
F:\inet\index.htm.
From then onwards they would navigate around the Intranet in the same way as they would get around the Internet.
Extranet
Extranet is an Intranet which is partially accessible to authorized outsiders users. In technical implementation server which hosts the which hosts the web pages remains behind a firewall which helps to control access between the Intranet and Internet permitting access to the Intranet only to authorized users.
The level of access can be set to different levels for individuals or groups of outside users. The access can be based on a username and password or an IP address.
Advantages
Exchange large volumes of data using Electronic Data Interchange (EDI)
Share product catalogs exclusively with external trade partners
Collaborate with other companies on joint development efforts
Jointly develop and use training programs with other companies
Provide or access services provided by one company to a group of other companies, such as an online banking application managed by one company on behalf of affiliated banks
Disadvantages
Extranets can be expensive to implement and maintain within an organization (e.g., hardware, software, employee training costs), if hosted internally rather than by an application service provider.
Security of extranets can be a concern when hosting valuable or proprietary information.
