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We've been using networks to model a variety of things.
We've used social networks to represent the friendships among people,
economic networks to capture the links between buyers and sellers.
And now we'd like to talk about a different type of network,
an information network which expresses logical relationships among pieces
of information.
The heart of this discussion will be sort
of a key example of an information network in modern society, which
is the worldwide web, something we use every day.
The web is an application built on the internet.
And it was essentially designed to solve a problem
to fill in something the internet lacked for the first two
decades of its existence.
All through the 1970s and 1980s, the internet
was good for allowing secure access to remote computers.
But there wasn't really an easy way to make files public to the whole world.
That changed rapidly in the year right around 1989, 1991,
when Tim Berners-Lee and a small team at CERN in Switzerland created the web.
The web based on three key ideas, really.
One, it provided an easy way to make documents,
the web pages, public for the whole world to see.
The second was that it gave users an easy way
to view these pages using browsers.
And the third, which is really the heart of our discussion here,
is that it was based on a way of connecting these pages using links.
And it was really this that turned the web into a network.
If you think about your own experience using the web,
this is sort of what you see, right?
Say you're reading a blog, you see a post on the blog,
you follow a link from that post to a news article,
you read that for a while, you back up, you read another post,
you follow a link there.
Although we don't always think about it this way, what you're really doing
is walking around on a big network where the nodes are pages
and the edges are links.
And you're following links, visiting other things,
going in different directions.
Let's see how this works with a picture.
Instead of thinking about these actual web pages
as they look on your browser screen, let's sort of
take a bird's eye view of them, or at least kind
of a conceptual view of them.
And say suppose we're starting at the home page for a course about networks.
And on this page, it has the various things
and it says we have a class blog where students write blog posts.
And it links to that blog.
And so we click on that link and we end up on the class blog
and there's a series of posts.
One of them is about Cornell, where the course is being taught.
One's about some company, we'll call it Company X. And maybe we're
interested in these, so we follow links out of them.
So the blog post about Company X leads to a news article about that company.
The post about Cornell actually takes us to the Cornell homepage.
And we explore around the Cornell homepage for a while
and we see they have a list of classes.
We go there.
And as we read down it, we see the way down in that list of classes
is a link to a course about networks.
And we follow that link and we actually end up back where we started.
So we walked around, maybe this took us five, 10 minutes,
we were aimlessly exploring.
But in the graph on this network, what we saw was a set of nodes
that we were traversing, and we were following these links
and we actually followed a path that brought us back to where we started.
Now because we're so familiar with this, from our experience,
we might think that, well, how else could you have organized the web?
This just seems like an obvious decision.
But in fact there are lots of ways that we organize documents.
We put them in folders, we put them in taxonomies or other organizations.
We could even sort them alphabetically.
In fact, structuring the web as a network
was both inspired and non-obvious.
It's really what gave it the kind of experience that we now see today.
And it really gives it its organizing principle.
But the idea to organize the web as a network didn't come out of nowhere.
It was really based on an idea called hypertext that had really
been pursued very passionately by a group of computer scientists
beginning with researchers like Ted Nelson in the 1960s.
The idea was to take text, the way we organize in books or articles,
and just sort of explode the linearity of that text.
Normally text just is one thing after another.
But hypertext was designed to break it apart and put into a conceptual space
where the links between the pieces of information could be made explicit.
Tim Berners-Lee and his team at CERN when
they created the web, Tim Berners-Lee had
been someone who read about hypertext.
He knew about hypertext and it was in his mind when he was building the web.
And so the architecture, the web was very much
based on this idea of exploding text and exposing the logical links explicitly.
And through the web, the idea of hypertext
was brought to a global audience on a scale
that really no one could ever have imagined.
But of course, the idea of networks on information and information
having network structure, that didn't start with the web or with hypertext.
It really draws on a bunch of intellectual precursors
that reach back to much earlier pre-technological eras.
And so these intellectual foundations for network structure and information
is something that drives how we think about the web and how it's organized.
And that's what we'll talk about next.