Virtual private networks (VPN) and IPsec are discussed in this chapter. A VPN emulates a private wide area network (WAN) facility using IP networks, such as the public Internet or private IP backbones. When VPNs are used, the Internet offers the appearance, functionality, and usefulness of a dedicated private network. One of the problems in using the Internet as a WAN is that the Internet is a public network and has relatively little security. IPsec provides the following security services to VPNs: data origin authentication, access control, confidentiality (encryption), connectionless integrity, rejection of replayed packets (a form of partial sequence integrity), and limited traffic flow confidentiality.