The Internetwork

 
Internetworking (a combination of the words inter ("between") and networking; it is not internet-working or international-network) is the practice of connecting a computer network with other networks through the use of gateways that provide a common method of routing information packets between the networks. The resulting system of interconnected networks is called an internetwork, or simply an internet.
The most notable example of internetworking is the Internet, a network of networks based on many underlying hardware technologies, but unified by an internetworking protocol standard, the Internet Protocol Suite, often also referred to as TCP/IP.
The smallest amount of effort to create an internet (an internetwork, not the Internet), is to have two LAN's of computers connected to each other via a router. Simply using either a switch or a hub to connect two local area networks together doesn't imply internetworking, it just expands the original LAN.

The definition of an internetwork today includes the connection of other types of computer networks such as personal area networks. The network elements used to connect individual networks in the ARPANET, the predecessor of the Internet, were originally called gateways, but the term has been deprecated in this context, because of possible confusion with functionally different devices. Today the interconnecting gateways are called Internet routers.

The Extranet

       
         An extranet is a computer network that allows controlled access from the outside, for specific business or educational purposes. In a business-to-business context, an extranet can be viewed as an extension of an organization's intranet that is extended to users outside the organization, usually partners, vendors, and suppliers, in isolation from all other Internet users. In contrast, business-to-consumer (B2C) models involve known servers of one or more companies, communicating with previously unknown consumer users. An extranet is similar to a DMZ in that it provides access to needed services for channel partners, without granting access to an organization's entire network.