Intranet: Intranet is a private internet network which is used by companies and organizations. All the common protocols like TCP/IP, HTTP, SMTP, etc. are supported by an intranet. Intranet is usually used to connect computers safely over a network and with simplicity or sharing files. Intranet can be used for email, discussion groups, access to private websites and databases, and teleconferencing.
LAN: Example of an intranet is LAN (Local Area Network). In a LAN, every computer doesn’t have to be connected individually. Only one computer can be connected to the internet and it serves as a gateway for other devices.
WAN: LANs can be interconnected to form a WAN (Wide Area Network). As the name suggests, WAN is used to connect computers in different locations. WANs are usually created by using leased lines. Routers are used to connect to the LANs and other computer systems.
Extranet: Extranet is a type of intranet which can be used to connect users in to a network using Internet. It is an extension of intranet where the network can be shared by people who are not actually the employees of an organization. In an extranet AAA (Authentication Authorization Accounting) protocol is used.