A virtual private network (VPN) is a technology for using the Internet or another intermediate network to connect computers to isolated remote computer networks that would otherwise be inaccessible. A VPN provides varying levels of security so that traffic sent through the VPN connection stays isolated from other computers on the intermediate network, either through the use of a dedicated connection from one "end" of the VPN to the other, or through encryption. VPNs can connect individual users to a remote network or connect multiple networks together.

For example, users may use a VPN to connect to their work computer terminal from home and access their email, files, images, etc.

Through VPNs, users are able to access resources on remote networks, such as files, printers, databases, or internal websites. VPN remote users get the impression of being directly connected to the central network via a point-to-point link.

An image of VPN

VPN systems can be classified by:

• the protocols used to tunnel the traffic
• the tunnel's termination point, i.e., customer edge or network-provider edge
• whether they offer site-to-site or remote-access connectivity
• the levels of security provided
• the OSI layer they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity

Security mechanisms
VPNs typically require remote access to be authenticated and make use of encryption techniques to prevent disclosure of private information.

VPNs provide security through tunneling protocols and security procedures such as encryption. Their security model provides:

• Confidentiality such that even if traffic is sniffed, an attacker would only see encrypted data which he/she cannot understand
• Allowing sender authentication to prevent unauthorized users from accessing the VPN
• Message integrity to detect any instances of transmitted messages having been tampered with

Secure VPN protocols include the following:

• IPSec (Internet Protocol Security) was developed by the Internet Engineering Task Force (IETF), and was initially developed for IPv6, which requires it. This standards-based security protocol is also widely used with IPv4. Layer 2 Tunneling Protocol frequently runs over IPSec. Its design meets most security goals: authentication, integrity, and confidentiality. IPSec functions through encrypting and encapsulating an IP packet inside an IPSec packet. De-encapsulation happens at the end of the tunnel, where the original IP packet is decrypted and forwarded to its intended destination.
• Transport Layer Security (SSL/TLS) can tunnel an entire network's traffic, as it does in the OpenVPN project, or secure an individual connection. A number of vendors provide remote access VPN capabilities through SSL. An SSL VPN can connect from locations where IPsec runs into trouble with Network Address Translation and firewall rules.
• Datagram Transport Layer Security (DTLS), is used in Cisco AnyConnect VPN, to solve the issues SSL/TLS has with tunneling over UDP.
• Microsoft Point-to-Point Encryption (MPPE) works with the Point-to-Point Tunneling Protocol and in several compatible implementations on other platforms.
• Microsoft's Secure Socket Tunneling Protocol (SSTP), introduced in Windows Server 2008 and in Windows Vista Service Pack 1. SSTP tunnels Point-to-Point Protocol (PPP) or Layer 2 Tunneling Protocol traffic through an SSL 3.0 channel.
• MPVPN (Multi Path Virtual Private Network). Ragula Systems Development Company owns the registered trademark "MPVPN".
• Secure Shell (SSH) VPN - OpenSSH offers VPN tunneling (distinct from port forwarding) to secure remote connections to a network or inter-network links. OpenSSH server provides a limited number of concurrent tunnels and the VPN feature itself does not support personal authentication.

Authentication
Tunnel endpoints must authenticate before secure VPN tunnels can be established.
User-created remote access VPNs may use passwords, biometrics, two-factor authentication or other cryptographic methods.
Network-to-network tunnels often use passwords or digital certificates, as they permanently store the key to allow the tunnel to establish automatically and without intervention from the user.