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Why Is a Network Tunnel Called a “Tunnel”?


The terminology one encounters when getting into the computing world may seem odd or leave you puzzled at times, wondering how and why these terms are in use. With that in mind, today’s SuperUser Q&A post has the answers to a confused reader’s questions.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader user7681202 wants to know why a network tunnel is called a “tunnel”:

I do not understand why the “tunnel” metaphor is used to describe a networking tunnel.

At first, I thought the reason was because the data was sent in an encrypted form so that an eavesdropper would not be able to see the data (seeing the data wrapped in a tunnel instead).

But what of the tunneling protocols that do not use encryption? Why are they also called “tunnels”?

Why is a network tunnel called a “tunnel”?

The Answer

SuperUser contributors Mokubai and DavidPostill have the answer for us. First up, Mokubai:

In the case of roads, a real world tunnel is a constructed passage that allows you to pass directly from point A to point B instead of taking a route that is longer and/or has more things to slow you down. Examples include tunnels through mountains that you might otherwise have to go around, underpasses that let you get to the other side of the road without walking across it, and subway tunnels that let trains move around a city without the need to contend with roads and buildings.

In each of these cases, a tunnel provides a direct path that avoids some type of complexity you would otherwise have to deal with. In networking, it is used in the same way.

An IPv4 over IPv6 tunnel allows IPv4 to pass through an IPv6 network into another IPv4 network, something that would otherwise not be possible without the originating computer understanding the IPv6 network.

A VPN is a tunnel specifically intended for connecting two private networks without the overhead of translating the IP addresses between private and public addresses at each end.

An example that combines the two is game VPN software like Hamachi that could be used to play “over the Internet” games which used old protocols like IPX or relied on local discovery to find other players.

Followed by the answer from DavidPostill:

Why is a network tunnel called a “tunnel”?

The phrase was first used (as far as I can tell) in the RFC 1075 Distance Vector Multicast Routing Protocol, where it is defined as follows:

And…

Although the above states, “We consider tunnels to be a transitional hack.”, tunneling is still used today with essentially the same meaning. The data sent through a tunnel is encapsulated so that it can be transmitted via a protocol that would otherwise not support the transmission:

Source: Networking 101 – Understanding Tunneling


Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Image Credit: mattthewafflecat (Pixabay)

Akemi Iwaya is a devoted Mozilla Firefox user who enjoys working with multiple browsers and occasionally dabbling with Linux. She also loves reading fantasy and sci-fi stories as well as playing “old school” role-playing games. You can visit her on Twitter and .


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