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A virtual private network (VPN) is a secure connection between two private networks over a public network (in other words, over the Internet). All the data that flows over the VPN is encrypted, so anyone who steals packets from the VPN will find them unintelligible; only the parties on either end of the VPN are able to decrypt the packets.

VPN connections are often called tunnels, because they provide an isolated pathway from one point to another through the Internet. The only way to gain meaningful access to a VPN tunnel is at either end.

There are two common uses for VPNs:

 To provide remote workers with secure access to your company network: To do that, you set up a VPN on the router, and then provide your remote workers with the credentials necessary to access the VPN. The remote workers can run a software VPN client on their home computers or laptops to connect to your company network.

 To establish a tunnel directly between routers on two networks that are separated geographically: For example, suppose you have offices in Los Angeles and Las Vegas. You can use routers on both networks to establish a VPN tunnel between them. This effectively joins the networks together, so that devices on the Los Angeles network can freely exchange packets with devices on the Las Vegas network, and vice versa.

Figure 3-4 shows this arrangement. As you can see, the routers in both Los Angeles and Las Vegas are connected through the Internet via a VPN tunnel. This tunnel enables computers in Los Angeles and Las Vegas to communicate freely and securely with each other.

FIGURE 3-4: Connecting offices with a VPN tunnel.

For more information about working with VPN tunnels, refer to Book 4, Chapter 6.