The domain name system it really doesn't 
have a great place to fit in this 
architecture. 
It is the thing that converts user 
friendly names like www.umich.edu to, to 
like a network friendly address. 
These network numbers and IP addresses 
are important, because they are the. 
they encode the geography of the, of the 
connections of the internet. 
But we humans really don't care about the 
geography of the interconnections of the 
internet. 
www.umich.edu or www.facebook.com is what 
we want to remember. 
So I like to kind of think of the, the 
the domain name system is kind of like, 
sort of, somewhere either between the 
internet and the transport, or between 
the internet and the link or somewhere 
like in this area. 
You know, somewhere here. 
And it's certainly not in the application 
layer, it's not the link layer. 
It uses the link layer. 
but basically, it's kind of a little add 
on to the side. 
So the domain name system is for 
user-friendly names, okay. 
So let's talk a little bit about just how 
it works, because domain names are what 
we use all the time and IP addresses are 
what computers use all the time. 
And routers use, routers really have no 
knowledge of the domain names, they 
simply move data based on the IP 
addresses. 
Numeric addresses are tough for people to 
remember. 
I remember in the old days we would have 
little post-it notes on our computers and 
that's how I would keep track. 
Oh, there's a new, Minnesota just put up 
a server, let's put that number on my 
little post-it note. 
So the fact that they were numbers didn't 
bother us at all, because there were only 
40 of em, but quickly there was more than 
40. 
And each campus even in the early days 
would only have one network number but 
now campuses have 20 or even 40 network 
numbers. 
Multiple groups of addresses, they're 
groups of addresses but their many of 
them. 
They get reorganized, you move a server 
from one place on campus to another like 
the, the Michigan web server, it was on 
our north campus so it would have one 
address, it was on our main campus. 
We'd have another reference and so you 
don't people to know what these IP 
addresses are. 
So they invented this notion of a domain 
name system, the. 
the visible name that we could switch the 
mapping from the name of the IP address 
transparently. 
And so the domain name system is like the 
internet's address book and it's a big 
distributed data base that's fast, it 
uses caching so that it's locally fast 
even if the network is partially down. 
IP addresses reflect the technical 
geography. 
And they read right to left, at least to 
the point where the network number is 
kind of the physical attachment point for 
the campus or business. 
And then, this part is the attachment 
point within that campus or business. 
And so, the most specific, the least 
specific to most specific goes from left 
to right. 
Doman names, on the other sa, hand are 
like organizational structure. 
The least specific is on the far right, 
and then it goes more specific as we go 
left. 
So edu means educational institutions, 
umich.edu means the particular 
educational institution University of 
Michigan, si.umich.edu means the school 
of information at the University of 
Michigan. 
Not all schools but just a particular 
one. 
And then dub, dub, dub is a particular 
server at that school. 
And it reads like a posta address. 
So, if you think that where I teach 2455 
North Quad, it's kind of on earth. 
And it's on the country USA, and it's in 
a zip code to the state of Michigan, it's 
in Ann Arbor. 
And then this is the building and then 
this is the room number in the building. 
So we go from very general to very 
specific. 
So, domain names lead, read in a sense 
right to left. 
Phone numbers and IP addresses read left 
to right. 
The other thing that's interesting about 
domain names is that they're owned. 
They're owned from right to left, and so 
there are sort of, they're a hierarchy, 
and basically an organization owns edu. 
The organization's name is Educause. 
It has conferences and other things but 
one of the things that it does is it owns 
in the public trust the name edu. 
So University of Michigan could go to edu 
and say hey could I have umich.edu. 
And they would say let me think about 
that, and then maybe they would give it. 
And they did, because University of 
Michigan, nobody else had it and it 
seemed like a good name and University of 
Michigan was like accredited in higher 
Education so they will give it to them. 
If I want to say like I'm Dr. 
Chuck and I want to create a Dr. 
Chuck University like Dr. 
Chuck.edu and they go like no, you can't. 
You're not a university. 
You're not a, you're not a educational 
institution. 
So they might say yes, they might say no. 
Things like .com and .org, they're kind 
of a catch as catch can and so the first 
one who gets there tends to win. 
There's, there's some situations where 
if, If I got cocacola.com, and had no 
particular use for it and Coca Cola owned 
the trademark Coca Cola, they could take 
it away from me, unless I had a 
legitimate purpose for it. 
It's hard for me to imagine a legitimate 
purpose for Coca Cola, but target.com 
might be a club for target shooting, and 
it would be difficult for Target to say, 
sorry I got to take your, your target 
shooting club away because thats', even 
though, though it's not as big as 
Target's comapny, it it's a legitimate 
use. 
So, at each level, once the University of 
Michigan is awarded the University of 
Michigan It creates a mechanism to a word 
subdomains within that. 
And so, there is actually committee at 
the University of Michigan. 
And if we want, like our learning 
measurement system is called 
ctools.umich.edu, if we want that, we 
have to go to the committee and convince 
them to give it to us. 
A top level domain within the University 
of Michigan. 
We got one, the School of Information, 
where I'm a faculty member, is 
si.umich.edu and they gave us that one. 
I can go to that committee and I could 
say hey, I want to be drchuck.umich.edu. 
And they would say no, you can't. 
So, then the school of information, where 
I teach has si.umich.edu and they have a 
committee. 
And they can give out sub-domains within 
that. 
So, you get the picture. 
Right? 
So, I could go there and I'd say, Hey, 
can I have drchuck.umich.edu, 
si.umich.edu? 
Drchuck.si.umich.edu. 
And it will say, No. 
But you can have csef. 
people.si.unich.edu. 
So once you own one of these things you 
can give out sub domains. 
I do own Drchuck.com. 
Drchuck.com And if there was a high 
desire for something underneath 
drchuck.com. 
That I could have my own little business 
selling domains under Dr. 
Chuck. 
com. 
nobody seems to want any of those things. 
But you know, some people have mail.com 
and other things and so each one of these 
is a new potential for expansion. 
And so, that's kind of how the domain 
name works, right, it is this mapping 
between these names There is sort of this 
right to left ownership of these names 
and and you have to ask people to get 
those names and so that's a real quick 
overview of the domain name system.