When you hear the word "tunneling," you presumably picture people excavating passageways beneath for something like a metro system or, if you're more scientifically inclined, a fiber optic cable link. However, tunneling in the context of computers refers to a protocol where one protocol is contained within another.

For creating different kinds of links over popular protocols, like HTTP, tunneling protocols are frequently used. To enable data to pass through a firewall, a peer-to-peer (P2P) file sharing application, for instance, may disguise data as HTTP. Since the Web utilizes port 80, which is supported by the majority of filters, the link would probably be permitted. The link might be stopped if the application used its own protocol and port number.

Additionally, tunneling can be used to create secure links over commonly insecure networks. For instance, the virtual private network (or VPN) between two distant places can be created using the point-to-point tunneling protocol (PPTP), which creates a safe link over a normal PPP connection. With the help of PPTP, a user can "tunnel" to a different place using a private password-protected link and the PPP protocol.