DHCP & NAT

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Del curso dictado por Princeton University

Networks Illustrated: Principles without Calculus

43 calificaciones

Princeton University

Networks Illustrated: Principles without Calculus

43 calificaciones

What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.

De la lección

Routing Traffic through the Internet

It is hard to overstate the impact that the Internet has had on society. In this lesson, we will overview the fundamental concepts behind the way the Internet is designed. We will also take a look at routing, which is the process of determining how packets of information are transported.

Conoce a los instructores

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So, now that we understand the ideas behind subnetting and IP addresses we

can dig even further to explain just two, two main notions that have developed.

they're both, they're both pretty simple and elegant but they're very important

and very, very, very fundamental to the way the Internet operates today.

the first one is DHCP.

host typically don't have fixed IP addresses, right?

So, you don't have a static IP address that'll

never change.

Every time you connect to the Internet, you are assigned a different IP address.

And you only can keep, you only keep that IP address

for a certain amount of time, unless you renew it, right?

And the Dynamic Host Configuration Protocol, or DHCP.

Now you know why we abbreviate everything because we don't want to

say, walk around saying things like

Dynamic Host Configuration Protocol all the time.

DHCP server keeps track of which addresses are unused, right.

So, it keeps like a library, or a database, of which addresses

it can assign that are currently unused, right?

And the DHCP server is typically in your

local network somewhere, that's assigning the IP addresses.

this the idea how it works.

there's some database that the DHCP server takes, is, is monitoring, right?

So, if your, when your device comes on the Internet and it says

okay, I want a, an IP address because I want to connect to the Internet.

The DHCP server will send this and it will say okay here's

an address that's that you can take 192.168.0.22.

And then you'd, you would have to say yes, I accept that address

and then you will be assigned this address so once you accept that.

And then the, the database will basically take this

address out of there for the time being.

but after some finite lease time which could be

any given amount of duration, just some amount of time.

The address is either going to be renewed, in which case you would

say yes, I'm still using this address, or it's returned to the database.

So, if you don't respond or you've disconnected

from the Internet and it, it's not your, it's,

the DHCP isn't receiving a response from this

address anymore it'll return this address to the database.

And same thing goes with all the other devices that are in your local network.

And that's how it keeps track of which

addresses are out and which addresses aren't being used.

So, the second idea is called NAT.

and this is another one, and the, the main idea behind NAT is that addresses in

your local network may be different actually from

addresses that you're seeing in, in the public Internet.

Alright, so we just, we just saw how how you get an IP address.

then NAT is basically to say well, your IP address from

your, viewed from your local colleagues, or your local devices that are

local to you might be different from what other people outside of you see.

And so, this is, this is, the idea this is

the Network Address Translation, NAT stands for Network Address Translation.

So, and NAT rather, does this.

And basically it translates back and

forth, it translate from public to private.

And this is the idea here, right? So, for the rest of the Internet, right?

So, suppose you have three devices and maybe this is in your home, right?

you, all your devices, all of the devices

may actually be on one public address, right?

So, all of these devices in your house may return and say okay when, when someone's

trying to address you, they may all be addressing 125, 125.35.48.166.

Then once it gets to the NAT router, the NAT router has some extra information

that it can use basically to say okay, I know where this needs to go.

I know which host it needs to go to.

And you actually use something beyond the IP address,

a little beyond our scope here, but you use port numbers, right?

So, each of you are running a different web browser

and using different port numbers for each of your sessions.

So, then the NAT will look and see, okay which port number it

was destined for, and then that's how it determines which host to go to.

So, everyone's addressing you again based upon this

public address, and then the NAT will be able

to convert it back to this private address, which

will then allow it to do the local networking.

so, the example of this is your home router, right?

so your home router, if you've ever seen before, your, your devices may be

10.0.25.1, 10.0.25.2, 10.0.25.3, and the router might just be 10.0.25.0.

So, for, you may see it like that.

But really, when everyone's addressing you outside, they're just

addressing your general IP address, your public address of 125.35.48.166.

And, the way to really think of

this a good analogy is a mail sorter in a large company.

so, you know like, if you have a mail sorter somewhere.

basically, what happens is, when someone addresses you, right?

they're going to write, basically, and say, okay, well, this

is going to go to Bob Smith in this company, right?

so then, the, the, the mail service will take it to the company, right?

And then a mail sorter would actually put it into Bob Smith's mailbox.

So, so, the NAT acts as this sorter.

And putting it in the mailbox, then it'll

look more specifically and have more information that it

needs to determine exactly which mailbox it goes into

or, for instance, which desk to deliver it to.

because it would get really tedious if all

the mail was delivered directly to someone's desk.

every time the mail truck came It would go to

every person's desk and deliver it one at a time.

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