Name: 2019年律師的CS50--互聯網技術、雲計算 (CS50 for Lawyers 2019 - Internet Technologies, Cloud Computing)
Uploaded: 2021-01-14T10:38:12.000Z
Duration: 1 h 14 min 21 s
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SPEAKER: You type an address into a browser, you send an email,

you perhaps have a video conference or a chat online.

Have you ever stopped to consider what exactly

is going on underneath the hood, so to speak, of those pieces of software?

And really the entire infrastructure that somehow

connects you to the person or persons with whom you're communicating.

Well it turns out that there's a whole stack, so to speak,

of internet technologies that underline the software that you and I use

And indeed, the software that we use, browsers

and email clients and the like, are really abstractions,

very user friendly abstractions, on top of some lower level implementation

And these days, too, have we built abstractions even above those so known

as the cloud, an abstraction on top of this underlying infrastructure

that enables us to do most anything we want computationally without even

So let's see if we can't distill what goes on when you do type an address

or a URL into the address bar of a browser and then hit Enter.

specify someone's email address and then hit Enter.

What exactly is going on underneath the hood?

Well, at the end of the day, I dare say that what your laptop, and my laptop,

and our desktops, and even our servers are capable of really

is just sending messages in envelopes back and forth across the internet.

Now in our human world, an envelope needs a few things on the outside.

If you want to send a letter or a card or something old school to someone,

And you need to put, perhaps in the middle, the recipient's name,

You might put in the top left hand corner, by convention, your own name

You might even put a little memo in the bottom

that specifies what's inside or fragile or some other annotation.

So this metaphor of the physical world is actually

pretty apt for what's going on underneath the hood in computers.

When you have a computer plugged into a network

or connected wirelessly to a network, it really

is just sending and receiving envelopes, virtual envelopes,

that at the end of the day are just patterns of zeros and ones,

but collectively, those zeros and ones represent your email or the request

that you've made of a web server, the response you're getting back

So let's see if we can't formalize exactly what these lower level

primitives are, consider exactly how they're layered on top of one another,

because thereafter we can build almost anything we want

on top of this infrastructure once we understand what those underlying

So let's consider how we actually address

After all, when I turn on my laptop or turn on my phone

or open up my desktop in the morning, how does that computer or that phone

even know what its own address is on the internet?

Because just as in our human world, wherein

you need to be uniquely addressable in the physical world

in order to even receive an envelope or a card or a package,

so do computers need to be uniquely identifiable on the internet.

Now for our purposes, now we can consider the internet just

to be an internetworked collection of computers connected

There's some kind of interconnectivity among all of these devices

and these days our phones and internet of things devices and other things

So let's just stipulate that somehow or other there's

a physical connection, or even a wireless connection,

So those devices all need unique addresses,

just like a building in the human world needs an address.

For instance, the computer science building here on campus is at 33 Oxford

Street, Cambridge, Massachusetts, 02138, USA.

With that precise information, can you send us a real mail or a package

or anything else through the physical world in order for it

But what if you, instead, wanted to send us an email

and get it to that building, or really me,

wherever I am physically in the world on my internet works device?

You need to know my computer's address, you

need to know my phone's address, or at least the mail server

that's responsible for receiving that message from you.

Well, it turns out that most any network on a campus, in a corporation,

even at home these days has a DHCP server.

Stands for a Dynamic Host Configuration Protocol,

and that's just a fancy way of describing a server that is constantly

listening for new laptops, new desktops, new phones, new other devices, to wake

up or be turned on and to shout out the digital equivalent of hello, world,

Because the purpose in life of these DHCP

To say, David you're going to go ahead and be address 1.2.3.4 today.

Or David, you're going to be 4.5.6.7 or 5.6.7.8.

Any number of possibilities can be used to represent

So DHCP servers are run by the system administrators on a campus,

in a company, in an internet service provider.

More generally, they're run by whoever provides us

But these DHCP servers also give us other information.

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2019年律師的CS50--互聯網技術、雲計算 (CS50 for Lawyers 2019 - Internet Technologies, Cloud Computing)

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