HTTP and HTML | Internet 101 | Computer Science | Khan Academy

HTTP and HTML | Internet 101 | Computer Science | Khan Academy


– I’m Jasmine Lawrence and I’m a Program Manager on the Xbox One engineering team. One of our biggest features
is called Xbox Live. It’s an online service
that connects gamers from all around the world, and we rely on the internet
to make that happen. This is no easy task and there are a lot of things happening behind the scenes. The internet is totally changing how people interact and connect. But how does it work? How do the computers all across the world actually communicate with each other? Let’s look at web browsing. First, you open a web browser. It’s the app you use to
access the web pages. Next, you type in the web address, or URL, which stands for Uniform Resource Locator, of a website you want to
visit like Tumblr.com. (upbeat electronic music) – Hi, I’m David Karp,
the founder of Tumblr, and we’re here today to talk about how those web browsers we use every day actually work. So you’ve probably wondered what actually happens when you type an address into your web browser and then hit enter, and it really is about as
crazy as you can imagine. So in that moment, your computer starts
talking to another computer called a server that’s usually thousands of miles away, and in milliseconds your computer asks that server for a website, and that server starts to talk back to your computer in a
language called HTTP. HTTP stands for Hypertext
Transfer Protocol. You can kind of think of it as the language that one computer uses to ask another computer for a document. It’s actually really
pretty straightforward. If you were to intercept the conversation between your computer and a web server on the internet, it’s mainly made up of something called “GET” requests. And those are really very
simply the word “GET” and then the name of the document that you’re requesting. So if you’re trying to log into Tumblr and load our login page, all you’re doing is sending a GET request to Tumblr’s server that says “GET /login” and that tells Tumblr’s server that you want all of the HTML code for the Tumblr login page. HTML stands for HyperText Markup Language and you can think of that
as the language you use to tell a web browser
how to make a page look. So if you think about
something like Wikipedia, which is really just
a big simple document, and HTML is the language that you use to make that title big and bold, to make the font the right font, to link certain text
to certain other pages, to make some text bold, to make some text italic, to put an image in the middle of the page, to align the image to the right, to align the image to the left. – The text of a web page is included directly in the HTML, but other parts, images or videos, are separate files with their own URLs that need to be requested. The browser sends separate HTTP requests for each of these and
displays them as they arrive. If a web page has a lot
of different images, each of them causes a
separate HTTP request and the page loads slower. Now sometimes when you browse the web, you’re not just requesting pages with GET requests. Sometimes you send information, like when you fill out a
form or type a search query. Your browser sends this information in plain text to the web server using an HTTP post request. – So let’s say you log in to Tumblr. The first thing you do
is make a “POST” request That is, a “POST” to Tumblr’s login page that has some data attached to it. It has your email address,
it has your password. That goes to Tumblr’s server. Tumblr’s server figures out
that, okay, you’re David. It sends a web page back to your browser that says, “Success! Logged in as David.” But along with that web page, it also attaches a little
bit of invisible cookie data that your browser sees and knows to save, and that’s really important because it’s really the only way that a website can remember who you are. All that cookie data really is is an ID card for Tumblr. It’s a number that
identifies you as David. Your web browser holds onto that number, and then the next time you refresh Tumblr, the next time you go to Tumblr.com, your web browser knows to automatically attach that ID number with the request that it sends over to Tumblr’s server, so now Tumblr’s server sees the request coming from your browser,
sees the ID number, and knows, okay, this
is a request from David. – Now, the internet is completely open, all of its connections are shared, and information is sent in plain text. This makes it possible for hackers to snoop on any personal information that you send over the internet, but safe websites prevent this by asking your web browser to communicate on a secure channel using something called
Secure Sockets Layer and its successor
Transport Layer Security. You can think of SSL and
TLS as a layer of security wrapped around your communications to protect them from
snooping or tampering. SSL and TLS are active when
you see the little lock that appears in your browser address bar next to the HTTPS. The HTTPS protocols ensure that your HTTP requests are secure and protected. When a website asks your browser to engage in a secure connection, it first provides a digital certificate, which is like an official ID card proving that it’s the
website it claims to be. Digital certificates are published by certificate authorities, which are trusted entities
that verify the identities of websites and issue
certificates for them, just like a government can
issue IDs or passports. Now, if a website tries to
start a secure connection without a properly issued
digital certificate, your browser will warn you. That’s the basics of web browsing, the part of the internet
we see day to day. To summarize, HTTP and DNS manage the sending and receiving of HTML, media files, or anything on the web. What makes this possible under the hood are TCP/IP and router networks that break down and transport information in small packets. Those packets themselves
are made up of binary, sequences of ones and zeros that are physically sent
through electric wires, fiber optic cables, and wireless networks. Fortunately, once you’ve
learned how one layer of the internet works, you can rely on it without remembering all the details. We can trust that all those
layers will work together to successfully deliver information at scale and with reliability.

8 thoughts on “HTTP and HTML | Internet 101 | Computer Science | Khan Academy”

  1. Endrank luvs da 4 loko

    This is really cool! It's pretty wild to think about all of the things that have to happen in a computer fto do something like watch a video on youtube. Great video! This entire computer series Khan Academy is posting is really great! Thanks

  2. CAs are 'tusted.'

    Not always. These are a plethora of CA you should disable for securities sake, say, on the average phone. Much less your home computer.

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