 So yesterday in class we talked a little bit about what the internet is. It's not Facebook. It's not Google. It's not Twitter It's the routers and the switches that make up the communications infrastructure So let's talk a little bit more today about what that means and how it's organized And we'll also look at some of the applications of the internet including email SSH and of course the World Wide Web being the most prominent example We'll take a little bit of a look and how the World Wide Web works behind the scenes and show you how you can Play around with it yourself Let's get started Now I'd like to show you a picture of what the internet looks like and rather than draw one I'll use the internet to look one up when you go to Google. You can click on this link images and do a Google image search Let's search for the internet And you can see a variety of pictures coming up Usually when I think of the internet I think of one of these constellation pictures in fact here's a picture from Wikipedia And if I just view that image You can see that it looks Like a constellation of the universe Basically, what you have in this picture is each one of these little dots. Let me zoom into this region right here Represents a computer connected to the internet and computers are connected to other computers Which are connected to routers and switches and ultimately the backbone of the internet itself The more densely connected things are in this picture the darker the color So the different shaded regions show you perhaps spanning different countries or going across different internet service providers I'm going to change my image search to internet service provider To give you something that's more of what you find in the book so here's an image that's similar to figure 4.7 where you have the internet consisting of Tier one providers or at least in this figure it says tier three. Let me see if I can find a better one Yeah, tier one ISPs and tier two networks which are mentioned in the book and ultimately all of us with our phones laptops and desktops are down here in this internet users portion of the Picture here we connect to our tier three networks such as Comcast or Verizon or Intel us and They connect into larger networks that are owned by different communications countries Sorry companies across the country those end up being the tier two networks in the tier three networks These little blue symbols all across the picture are routers, right? And so they're not even Typically a router could be a computer or it can be specialized hardware But its sole purpose is basically to take bits or packets and transfer them from one end of the router to the appropriate other end depending on where the destination of the packet is going Now as I mentioned before as we looked at the constellation picture each one of the computers on the internet has an IP address IP stands for internet protocol It's the address that says this is where a packet is going to or the address where a packet is coming from Let's take a look at an IP address example Here's some now most of the computers are still using IP version 4 which is a 32 bit address You can see here 32 ones and zeros that represent the address of a computer now Just like we learned to write hexadecimal to reduce the number of zeros and ones a person has to look at There's another way to organize these binary digits in what's called dotted decimal notation if we take each eight bits and turn it into a decimal decimal number like 172 16 and so forth Then we have four Decimal numbers the decimal numbers range from zero to 255 because that's how many different numbers you can represent with eight bits 256 or 2 to the 8th power so basically this is a typical IP address 4 decimal numbers separated by dots which makes up 32 bits Let's take a look at what the IP address of my machine is I'm going to hit control plus a couple times to make the font bigger Now on Linux or Mac the command to see your IP address is if config This means internet or sorry interface config for network interface, and you'll see a variety of different Things output so one is my internet address, and this happens to be the address of my desktop at work and Also a hardware address which is a series of hexadecimal digits that encodes the serial number of my network card Pretty much every network interface in any device has a unique hardware address I Also see an internet version 6 address which is a 128 bit address and that's beyond the scope of this class But the internet is running out of addresses, so we're moving to a new version of IP that allows you more bits to uniquely identify each device I Also know how many packets I've received and how many packets I've transmitted and so forth Now as we know human beings aren't very good at remembering large numbers or large groups of numbers instead We'd like to refer to a computer by name such as going into a web browser and typing in the address google.com Well Google.com has an IP address look just like anything else And if you want to see the IP address of a particular site you can use the host command host google.com and This gives me the different set of addresses that Google.com has Google is such a large service that it needs to have many IP addresses to balance the load across all of its different users But I could have just as well taken this IP address copy it and paste it into my web browser and It works the same way whether I use the name or the IP address directly Let's do another one host jmu.edu So jmu.edu only has one IP address because it's not quite so big as Google this process of converting a domain name like jmu.edu to an IP address like 134.126.10.50 is called domain name system You can read about it for example on Wikipedia or in your book and the basic idea is you send a request To a domain name server that will tell you oh, I know the address of Google.com. It is 216.12.120.50 277 the host command actually connects to a domain name server to get you the IP address of a host name There's other things you can do with DNS for example. I can run the who is command and Who is will get me the registration information for that domain? So for example the registrant of jmu.edu is James Madison University. Here's the address. Here's the contact person and Their phone number and their email address and so forth Let's see who owns google.com who is google.com and The output's a lot longer here You can contact with an address or a phone number or a fax if you're back in the 80s I guess and a bunch of other information So basically any website that's registered has to have some of this information like a contact person or an address or Somebody to complain to if the site is misbehaving for example Again, that's the who is command to see who a site is. Let's do yahoo.com Or you can use the host command To actually get the IP address of the host. These are different commands that allow you to interact with the domain name system