An Intranet is a private website (or websites) on a private network. Only users inside of an organization can access such a service. When an Intranet website is available externally of the organization, the outside users are interacting with an extranet.
An Intranet server is assigned a private LAN IP address such as a 10.x.x.x, 172.16.x.x, or 192.168.x.x IP address. Such an IP address creates a "closed" environment from the public Internet. The closed network environment allows employees only service(s) for users that are affiliated with the organization.
The IP Protocol standard prevents all private IP addresses from leaving the private network as well as prevents private IP addresses from entering the network from an ISP. Local IP packets heading to the Internet undergo IP Masquerading to leave the private network.
Intranets are put to use in companies, schools and public organizations. By keeping information local, a company can, for example, create a private wiki containing trade secrets or a school may keep copyrighted academic material protected from the public within the materials' license rights.
A virtual private network VPN is an extension of an organization's network that is unseen to the public Internet. A user can interact with an Intranet service by connecting his or her own personal network or computer to the private network of the Intranet equipped organization via the Internet.
- Intranet Wikipedia Entry (en.wikipedia.org)
- RFC 1597 on Address Allocation for Private Internets (ftp.rfc-editor.org)
- IP Masquerade HOWTO (tldp.org)