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Posted: 03 May 2013 02:58 AM PDT

There are three basic VPN categories that describe where a VPN is used.

Intranet
Extranet
Internet

Intranet - A VPN connection between same companies. Transport mode connections within a company’s infrastructure, such as VPN between two devices (a router sending traffic to a sys log server, a PIX appliance backing up its configuration to a TFTP server, and a user on a PC using Telnet to a Catalyst 3550 switch, to name a few). Tunnel mode connections between different locations within a company’s infrastructure, such as VPN between two offices via a private Frame Relay or ATM network

Extranet - A VPN connection from one company to another company, such as a business partner. An example of an extranet could be a company that has outsourced its help desk functions and sets up a VPN to provide a secure connection from its corporate office to the outsourcing company.

Internet - An Internet VPN uses a public network as the backbone to transport VPN traffic between devices. As an example, you might use the Internet, which is a public network, to connect two sites together (L2L connection), or have telecommuters use their local ISPs to set up a VPN connection to the corporate network (remote access connections).

VPN Tunnelling

Tunneling is the most significant component of VPN technology. Tunneling is the technique of encapsulating an entire data packet in the packet of another protocol format. When a tunneled packet is routed to the destination node, it travels across the internet work through a logical path. This logical path is referred to as a tunnel. Tunneling is analogous to sending a letter. After you write a letter, you place it in an envelope. This envelope displays the address of the recipient. When you post this letter, it is delivered to the recipient according to the address on the envelope. The recipient then needs to open the envelope to read the letter. In tunneling technology, the letter is equivalent to the original payload and the envelope represents the packet of the routable protocol in which the payload is encapsulated. The address on the envelope represents the routing information that is appended to the packet.

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