A virtual private network (VPN) is a system that delivers enterprise-focused communication services on a shared public network infrastructure and provides customized operating characteristics uniformly and universally across an enterprise. The term is used generically to refer to voice VPNs. To avoid confusion, IP-based data services are referred to as data VPNs. Service providers define a VPN as a WAN of permanent virtual circuits, generally using asynchronous transfer mode (ATM) or frame relay to transport IP. Technology providers define a VPN as the use of encryption software or hardware to bring privacy to communications over a public or untrusted data network.