Virtual Private Network (VPN) refers to a privately used network that built on a shared network infrastructure. Each company seems to be using the network "privately", but in fact the network is shared by many companies, so the term "virtual" is used. Most layer 2 shared networks like ATM and Frame Relay are supporting VPN services. In the recent decade Internet Protocol (IP) becomes the dominant communication protocol supporting multiple applications, so, most VPNs are in fact running IP on it and hence they are called IP VPN.

About the evolution of IP VPN, from the 80's using "international private leased circuits (IPLC)" to interconnect multiple sites, to the 90's using ATM/FR to build the IP VPN, and in the late 90's, when the internet is getting popular and having extended coverage, some companies build their IP VPN over the public internet. Until year 2000, with the standardization of Multi Protocol Label Switching (MPLS) standard and the conforming equipment being shipped from major vendors, the latest trend of high performance, high quality IP VPN is MPLS IP VPN.