Virtual Private Networks, VPN
A Virtual Private Network (VPN) allows communication between two devices located on different subnets, such that each appears to the other as though it is on the same subnet. For example, an employee working from home would be able to access an office printer as though it were connected to the home computer. In effect, the office private network is extended, over the public internet, to the home-worker's private network.
In order to prevent third parties from monitoring the data sent over the VPN, the content is encrypted. In addition, steps must be taken to ensure that only authorised machines can connect to the VPN.
To create a VPN connection over the internet, tunneling software must be run at each end of the connection which transparently re-routes packets addressed to a local IP, so that they are forwarded to the remote IP before being sent to the relevant resource on the remote network. The response is then similarly forwarded back to the original machine and made to appear as though it came from a local resource.
The sequence of operations for a client requesting data from a remote server over a VPN would be as follows:
The overall effect is that the client communicates securely with the remote resource as though it were connected locally.
See also: Private IP Addresses.