IFSM 310 – Cloud Computing vs. Client-Server Computing - DUE FRIDAY

Chapter 5

The Domain Name System

The Domain Name System lets you access websites by their

domain name like (www.khanacademy.org), so you don’t have

to keep a list of numeric Internet Protocol (IP) addresses like

“212.78.1.25”. IP address are determined by where your com-

puter connects to the Internet. When you have a portable

computer and you move from one location to another, you get

a new IP address at each new location. Since no one connects

to your portable computer, it does not matter if your IP address

changes from time to time. But since so many people connect to

a web server, it would be inconvenient if the server moved to a

new location and needed to change its IP address.

When your computer makes a connection to a system using a

domain name address, the first thing your computer does is look

up the IP address that corresponds to the domain name. Then

your computer makes the connection using the IP address.

Adding the separate step of looking up the IP address for a DNS

address also makes it easier to move a server from one location

to another. The server is given a new IP address and the entry for

the domain address is updated. Once the DNS entry is updated,

new requests for the domain name are given the new IP address.

Since end users access most servers using domain names and

never see the IP address, a server can be moved to a new network

connection without affecting the end user’s ability to access the

server.

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58 CHAPTER 5. THE DOMAIN NAME SYSTEM

5.1 Allocating Domain Names

If you recall from the previous section, IP addresses are allocated

based on where you connect a new network to the Internet.

Domain names are allocated based on organizations that “own”

the domain name. At the top of the domain name hierarchy is

an organization called the International Corporation for Assigned

Network Names (ICANN). ICANN chooses the top-level domains

(TLDs) like .com, .edu, and .org and assigns those to other

organizations to manage. Recently a new set of TLDs like .club

and .help have been made available.

ICANN also assigns two-letter country code top-level domain

names like .us, .za, .nl, and .jp to countries around the world

We call these Country-Code Top-Level Domain Names (ccTLDs).

Countries often add second-level TLDs, like .co.uk for commercial

organizations within the UK. Policies for applying for domain

names with any particular ccTLD vary widely from one country to

another.

Figure 5.1: Domain Names

Once a domain name is assigned to an organization, the con-

trolling organization is allowed to assign subdomains within the

domain. As an example, the .edu top-level domain is assigned

5.2. READING DOMAIN NAMES 59

to the Educause organization. Educause assigns domains like

umich.edu to higher education institutions. Once the University

of Michigan is given control of umich.edu, it can make its own

choices for subdomains within its new domain. Domains ending

in .com and .org can be purchased by individuals. The individual

owners of those domains are allowed to manage their domain and

create subdomains under it for their own use or use by others.

5.2 Reading Domain Names

When we look at an IP address like “212.78.1.25”, the left prefix

is the “Network Number”, so in a sense we read IP addresses

from left to right, where the left part of the IP address is the most

general part of the address and right part of the address is most

specific:

212.78.1.25

Broad ----> Narrow

For domain names, we read from right to left:

drchuck.personal.si.umich.edu

Narrow <--- Broad

The most general part of this domain name is “.edu”, which

means higher education institutions. The subdomain “umich.edu”

is a particular higher education institution.

5.3 Summary

While the Domain Name System is not one of our four layers in

the model, it is an important part of making the Internet easier to

use. Domain names allow end users to use symbolic names for

servers instead of numeric Internet Protocol addresses. By adding

a service that maps domain names to IP addresses, we can move

servers from one Internet connection to another connection with-

out requiring users to manually change their configurations to

connect to a server.

If you would like to purchase a domain name for yourself or your

company, you can choose from any number of domain name reg-

istrars.

60 CHAPTER 5. THE DOMAIN NAME SYSTEM

5.4 Glossary

DNS: Domain Name System. A system of protocols and servers

that allow networked applications to look up domain names and

retrieve the corresponding IP address for the domain name.

domain name: A name that is assigned within a top-level do-

main. For example, khanacademy.org is a domain that is assigned

within the “.org” top-level domain.

ICANN: International Corporation for Assigned Network Names.

Assigns and manages the top-level domains for the Internet.

registrar: A company that can register, sell, and host domain

names.

subdomain: A name that is created “below” a domain

name. For example, “umich.edu” is a domain name and

both “www.umich.edu” and “mail.umich.edu” are subdomains

within “umich.edu”.

TLD: Top Level Domain. The rightmost portion of the domain

name. Example TLDs include “.com”, “.org”, and “.ru”. Recently,

new top-level domains like “.club” and “.help” were added.

5.5 Questions

You can take this quiz online at http://www.net-intro.com/quiz/

1. What does the Domain Name System accomplish?

a) It allows network-connected computers to use a textual

name for a computer and look up its IP address

b) It keeps track of the GPS coordinates of all servers

c) It allows Regional Internet Registries (RIRs) to manage IP ad-

dresses on the various continents

d) It assigns different IP addresses to portable computers as

they move from one WiFi to another

2. What organization assigns top-level domains like “.com”,

“.org”, and “.club”?

a) IANA - Internet Assigned Numbers Authority

b) IETF - Internet Engineering Task Force

5.5. QUESTIONS 61

c) ICANN - International Corporation for Assigned Network

Names

d) IMAP - Internet Mapping Authorization Protocol

3. Which of these is a domain address?

a) 0f:2a:b3:1f:b3:1a

b) 192.168.3.14

c) www.khanacademy.org

d) @drchuck

4. Which of these is not something a domain owner can do with

their domain?

a) Create subdomains

b) Sell subdomains

c) Create new top-level domains

d) Assign an IP address to the domain or subdomain

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