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

these days, every day.

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

details.

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

having that hardware locally.

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.

Or you type out an email--

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.

Virtual envelopes, if you will.

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,

you need to address it, of course.

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

and address, and other details.

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

and or address.

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

from that web server.

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

building blocks actually are.

So let's consider how we actually address

this envelope in the first place.

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

via wires, connected wirelessly.

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

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

still.

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

a physical connection, or even a wireless connection,

between all of these various devices.

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

to arrive on our doorstep?

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,

what is my address?

Because the purpose in life of these DHCP

servers is to answer that question.

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

uniquely my particular device.

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

with our internet connectivity.

They just exist on our network.

But these DHCP servers also give us other information.

After all, it's not really sufficient just to know what my own address is.

How do I know where anyone else in the world is?

Well, it turns out that the internet is filled

with devices called routers whose purpose in life,

as their name suggests, is to route information from point A

to point B to point C and so on.

And those routers, similarly, need to know these addresses

so that they know upon receiving some packet of information,

some virtual envelope, in which direction to send it off.

So these DHCP servers also tell me not just my address, but also the address

of the next hop, so to speak.

I, as a little old laptop or phone or a desktop,

I have no idea where 99.999 percent of the computers in the world

are, even higher than that perhaps.

But I do need to know where the next computer is on the internet,

so that if I want to send information that leaves this room,

it needs to go to a router whose purpose in life

is to, again, route it further along.

And generally there might be one, two, maybe even 30 steps

or hops in between me and my destination for that email or virtual envelope,

and those routers are all configured by people who aren't me,

system administrators beyond this, beyond these walls

to know how to route that data.

So we can actually see evidence of this that you yourself

have had underneath your fingertips all this time

and you might not have ever poked around.

For instance, if you want to see your own address,

keep an eye out for a number of this form.

It's a number dot number dot number dot number, and each of those place holders

represents a specific value, either starting at zero or ending at 255.

In other words, each of these hashes can be any value between 0 and 255,

and that range 0 to 255 well that's 256 total possible values.

That's eight bits.

Ergo, each of these place holders represents 8 bits, 8 more bits, 8 more,

8 more.

So an IP address, by definition, is 32 bits.

And there it is.

IP, an acronym you've probably seen somewhere,

even if you've not thought hard about what it is,

stands for Internet Protocol.

Internet Protocol mandates that every computer on the internet,

at the risk of oversimplification, has a unique address called an IP address.

And those IP addresses look like this.

If these IP addresses are composed of 32 bits, how many possible IPs are there

and therefore how many possible machines can we have on our internet?

Well, 2 times 2 times 2, 2 to the 32, so that's four billion, give or take.

By design of IP addresses, you can have four billion,

give or take, possible permutations of zeros and ones if you have 32 in total,

and that gives you four billion, maximally,

computers and phones and internet of things devices, and the like.

Now that sounds big, but not when each of us

personally probably carries one IP address in our pocket in our phone,

maybe another on our wrist these days, one or more computers

in our life, not to mention all of the other devices and servers in the world

that need these addresses, too.

So long story short, this is version 4 of IP.

It's decades old, but there's also a newcomer on the field

called IPv6, version 6.

There isn't really to be a version 5.

And IPv6 is only finally gaining traction

because we're running so short on IPs that it's

becoming a problem for campuses, for companies, and beyond.

But IPv6 will use 128 bits instead of 32,

which gives us many, many, many, many, more possibilities, bigger

numbers than I can even pronounce.

So that should cut it for quite some time.

But not every computer on the internet needs a public IP address, only

those envelopes, so to speak, that need to leave my pocket, or my home,

or my campus, or my company.

It turns out, as a short term mechanism to squeeze a bit more utility out

of our 32-bit addresses, which are still omnipresent

and the most popular among the versions, well

we can actually distinguish between public IP

addresses that do actually go out on the internet and private addresses.

And indeed, if your own IP address happens

to start with the number 10 and then a dot or the number

172.16 and then a dot, or the number 162.168 and then a dot,

and then something else, well, odds are, your computer has a private IP address.

And this is just a feature of the little router that's probably in your home,

or the bigger router on your campus or corporate network,

that enables you to have an IP address that's only used within the company,

only used within your home, and cannot, by definition,

be routed publicly beyond your company, beyond your home,

because the router will stop it.

And so here we actually have the beginnings of a firewalling mechanism,

if you will.

In the real world, a firewall is a device

that prevents fire from going from one store to another, for instance.

In the virtual world, a firewall is a piece of software

that prevents zeros and ones from going from one place to another.

And in this case do we already have a mechanism

via public and private addresses of keeping some data securely,

or with high probability securely, within our company

versus allowing it to go out on the internet.

So we'll see now some screenshots of some actual computers from Mac OS

and Windows alike that reveal their IP addresses,

and you yourself can see this on your own machines.

For instance, here on Windows 10 is a screenshot

of what your Network Preferences, so to speak, might look like.

And if you focus down here, it's a bit arcane at first glance,

but IPv4 address is 192168.1.139 when we took that screenshot.

And indeed, it starts with 192168 which means it's private,

and indeed, I took this screenshot while we were within a home network,

and so that suggests it can be used to route among computers in that home

but not beyond.

You'll see, too, if we move on to the next screen

where you see more advanced network properties,

you can also see the dimension of this default gateway, which is

synonymous with router, default router.

192168.1.1.

So a default router or default gateway is that first hop,

so that if I want to send an email outside of my home,

I want to visit a web page outside of my company,

all I need do is hand that virtual envelope containing that email

or that web request off to the machine on the local network that

has that IP address.

I have no idea where it's going to go thereafter, to hops two and three

and beyond, but that's why we have this whole internet

and even more routers out there.

They, the routers, intercommunicate and relay that data,