Deﬁnition: A virtual private network (VPN) is a private network that protects communications sent through a public network, such as the Internet. VPNs provide secure connections between endpoints, such as routers, clients, or servers, by using tunneling to encrypt data. These connections are established either between two LANs or between a user and a LAN over the Internet. Special VPN protocols are required to provide the VPN tunneling, security, and data encryption services.
Figure: A VPN.
An Ethernet network is a popular LAN implementation that uses Ethernet network adapters, contention-based media access, and twisted pair, coax, or ﬁber media. Xerox corporation ﬁrst developed Ethernet in the 1970s. Later, the IEEE used Ethernet as the basis of the 802.3 speciﬁcation, which standardized Ethernet and expanded it to include a wide range of cable media. The 802.3 family of speciﬁcations also determines transmission speed (10 Mbps, 100 Mbps, or 1000 Mbps) and signal method (baseband or broadband).
Contention-based Media Access
Ethernet networks use contention to control how stations access the media. This means that each station must compete with others for an opportunity to transmit. If a station sends data and then detects a data collision, the station must wait for a backoff period before it sends again. This particular contention scheme is known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Contrast this with controlled media access schemes, in which stations receive explicit permission to transmit, for example, by possession of an electronic token, as in Token Ring networking.