What is VPN ? ( Virtual Private Network)

Virtual private network

A virtual private network (VPN) is a computer network in which some of the links between nodes are carried byopen connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The link-layer protocols of the virtual network are said to be tunneled through the larger network when this is the case. 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 topology more complex than point to point. The distinguishing characteristics 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.

A typical VPN might have a main LAN  at the corporate headquarters of a company, other LANs at remote offices or facilities and individual users connecting from out in the field.
Basically, a VPN is a private network that uses a public network (usually the Internet) to connect remote sites or users together. Instead of using a dedicated, real-world connection such as leased line, a VPN uses “virtual” connections routed through the Internet from the company’s private network to the remote site or employee. In this article, you will gai­n a fundamental understanding of VPNs, and learn about basic VPN components, technologies, tunneling and security.

­A well-designed VPN can greatly benefit a company. For example, it can:

• Extend geographic connectivity
• Improve security
• Reduce operational costs versus traditional WAN
• Reduce transit time and transportation costs for remote users
• Improve productivity
• Simplify network topology
• Provide global networking opportunities
• Provide telecommuter support
• Provide broadband networking compatibility
• Provide faster ROI (return on investment) than traditional WAN

What features are needed in a well-designed VPN? It should incorporate:

• Security
• Reliability
• Scalability
• Network management
• Policy management

Business Case for Using VPN

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. 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.