VPN (Virtual Private Network)

A virtual private network (VPN) is a communications networkprotocol" tunneled through another network, and dedicated for a specific network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.
A VPN may have best-effort performance, or may have a defined Service Level Agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a more complex than point-to-point. The distinguishing characteristic of VPNs are not security or performance, but that they overlay other network(s) to provide a certain functionality that is meaningful to a user community.
Attractions of VPNs to enterprises include:
Shared facilities may be cheaper—especially in capital expenditure (CAPEX)—than traditional routed networks over dedicated facilities.
Can rapidly link enterprise offices, as well as small-and-home-office and mobile workers.
Allow customization of security and quality of service as needed for specific applications.
Can scale to meet sudden demands, especially when provider-provisioned on shared infrastructure.
Can reduce operational expenditure (OPEX) by outsourcing support and facilities.
Distributing VPNs to homes, telecommuters, and small offices may put access to sensitive information in facilities not as well protected as more traditional facilities. VPNs need to be designed and operated under well-thought-out security policies. Organizations using them must have clear security rules supported by top management. When access goes beyond traditional office facilities, where there may be no professional administrators, security must be maintained as transparently as possible to end users.
Some organizations with especially sensitive data, such as health care companies, even arrange for an employee's home to have two separate WAN connections: one for working on that employer's sensitive data and one for all other uses. citation needed More common is that bringing up the secure VPN cuts off Internet connectivity for any use except secure communications into the enterprise; Internet access is still possible but will go through enterprise access rather than that of the local user.
In situations in which a company or individual has legal obligations to keep information confidential, there may be legal problems, even criminal ones, as a result. Two examples are th regulations in the U.S. with regard to health data, and the more general European Union data privacy regulations which apply to even marketing and billing information and extend to those who share that data elsewhere.
The Internet Engineering Task Force (IETF) categorized a variety of VPNs, some of which, such as Virtual LANs (VLAN are the standardization responsibility of other organizations, such as the Institute of Electrical and Electronics Engineers (IEEE) Project 802, Workgroup 802.1 (architecture). Originally, network nodes within a single enterprise were interconnected with (WAN) links from a telecommunications service provider. With the advent of LANs, enterprises could interconnect their nodes with links that they owned. While the original WANs used dedicated lines and layer 2 multiplexed services such as Frame Relay, IP-based layer 3 networks, such as the ARPANET, Internet, military IP networks (SIPRNET,JWICS, etc.), became common interconnection media. VPNs began to be defined over IP networks. The military networks may themselves be implemented as VPNs on common transmission equipment, but with separate encryption and perhaps routers.
It became useful first to distinguish among different kinds of IP VPN based on the administrative relationships, not the technology, interconnecting the nodes. Once the relationships were defined, different technologies could be used, depending on requirements such as security and quality of service.
When an enterprise interconnected a set of nodes, all under its administrative control, through an IP network, that was termed an Intranet When the interconnected nodes were under multiple administrative authorities, but were hidden from the public Internet, the resulting set of nodes was called an extranet. Both intranets and extranets could be managed by a user organization, or the service could be obtained as a contracted offering, usually customized, from an IP service provider. In the latter case, the user organization contracted for layer 3 services much as it had contracted for layer 1 services such as dedicated lines, or multiplexed layer 2 services such as frame relay.
The IETF distinguishes between provider-provisioned and customer-provisioned VPNs Much as conventional WAN services can be provided by an interconnected set of providers, provider-provisioned VPNs (PPVPNs) can be provided by a single service provider that presents a common point of contact to the user organization.

Tunneling protocols can be used in a point-to-point topology that would generally not be considered a VPN, because a VPN is accepted to support arbitrary and changing sets of network nodes. Since most router implementations support software-defined tunnel interface, customer-provisioned VPNs are often simply a set of tunnels over which conventional routing protocols run. PPVPNs, however, need to support the coexistence of multiple VPNs, hidden from one another, but operated by the same service provider.
Building Blocks
Depending on whether the PPVPN is layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combinations of the two. MPLS functionality blurs the L2-L3 identity.
While these terms were generalized to cover L2 and L3 VPNs in RFC 4026, they were introduced in .
Customer Edge Device (CE)
In general, a CE is a device, physically at the customer premises, that provides access to the PPVPN service. Some implementations treat it purely as a demarcation point between provider and customer responsibility, while others allow it to be a customer-configurable device.
Provider Edge Device (PE)
A PE is a device or set of devices, at the edge of the provider network, which provides the provider's view of the customer site. PEs are aware of the VPNs that connect through them, and do maintain VPN state.
Provider Device (P)
A P device is inside the provider's core network, and does not directly interface to any customer endpoint. It might, for example, be used to provide routing for many provider-operated tunnels that belong to different customers' PPVPNs. While the P device is a key part of implementing PPVPNs, it is not itself VPN-aware and does not maintain VPN state. Its principal role is allowing the service provider to scale its PPVPN offerings, as, for example, by acting as an aggregation point for multiple PEs. P-to-P connections, in such a role, often are high-capacity optical links between major locations of provider.