What is VPN
A VPN is a logical network created on top of another network, such as the Internet. Despite the fact that communications are carried out over public networks using insecure protocols, due to encryption, closed channels of information exchange are created.

VPN allows you to combine, for example, several offices of the organization into a single network using uncontrolled channels for communication between them. At its core, a VPN has many of the properties of a dedicated line, but it is deployed within a public network, such as the Internet. Using the tunneling technique, data packets are broadcast over the public network as over a conventional point-to-point connection.

Between each pair of "sender-receiver data" is established a kind of tunnel-a secure logical connection that allows you to encapsulate the data of one Protocol in the packets of another. The main components of a tunnel are: initiator; routed network; tunnel switch; one or more tunnel terminators.

By itself, the principle of VPN does not contradict the basic network technologies and protocols. For example, when a remote access connection is established, the client sends a stream of standard PPP packets to the server. In the case of virtual dedicated lines between local networks, their routers also exchange PPP packets. However, a fundamentally new point is the forwarding of packets through a secure tunnel organized within a public network.

Tunneling allows you to transfer packets from one Protocol to a logical environment that uses a different Protocol. As a result, it is possible to solve the problems of interaction of several different types of networks, starting with the need to ensure the integrity and confidentiality of the transmitted data and ending with overcoming inconsistencies of external protocols or addressing schemes. The existing network infrastructure of the Corporation can be prepared for the use of VPN with both software and hardware.

Organizing a virtual private network can be compared to laying a cable through a wide area network. Typically, the direct connection between the remote user and the tunnel endpoint is established via PPP. The most common method of creating VPN tunnels is to encapsulate network protocols (IP, IPX, AppleTalk, etc.) in PPP and then encapsulate the resulting packets in a tunneling Protocol. Usually the latter is IP or (much less often) ATM and Frame Relay.

This approach is called second-level tunneling, because the "passenger" here is the second-level Protocol. An alternative approach-encapsulating network Protocol packets directly into a tunneling Protocol (such as VTP) is called third-level tunneling.

Regardless of what protocols are used or what goals are pursued in the organization of the tunnel, the basic methodology remains virtually unchanged. Typically, one Protocol is used to establish a connection to a remote host, and the other is used to encapsulate data and service information for transmission through a tunnel.