Structure of the Internet: Domain names

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UNIT 2 - ⇑ Structure of the Internet ⇑

← IP addresses Domain names URIs →

Domain Name - humanly-memorable names for Internet participants such as computers, networks, and services. One domain name can be connected to multiple IP addresses


Trying to remember IP addresses is as difficult as trying to remember people's phone numbers. Not many people do it well and you are far more likely to be using a domain name to access a website. A domain name allows us to link to servers and other computers using easily remembered names. The domain name also tells us a bit about the location we are visiting through the use of top level domain names.

Top level domain Description
.com Commercial
.gov US governmental organisations
.mil US military
.org Organisation
.uk United Kingdom, country specific website
.so Somalia, country specific website
.xxx adult entertainment

Sometimes top level domain names might be joined by second level domains, chained together to tell you more detailed information:

bbc.com - there is a commercial organisation called bbc
bbc.co.uk - there is a United Kingdom commercial organisation called bbc.
tfl.gov.uk - there is a United Kingdom governmental organisation, called TFL.

Second level domain names typically tell you the person/company/organisation that owns the address. Finally you have the host or service name, which is the machine/service you are connecting to. For example:

en.wikipedia.org - an organisation, called Wikipedia, requesting the English language version
www.ibm.com - an commercial organisation, called IBM, connecting to the www (World Wide Web) host/service machine
\underbrace{windows}_\text{host} . \underbrace{microsoft}_\text{domain name} . \underbrace{com}_\text{top level domain}
Diagram of how domain names are constructed

In the example above a client sends a request to a Domain Name Server for www.debian.com, but after that domain name lookup is hierarchical:

  • a request for .com is first sent to a known root server which provides the address of the name server for .com.
  • This is then queried for debian.com giving the address of another name server
  • which finally provides the address for www.debian.com.

Results are remembered for a period of time to reduce the work load.

Domain Name Server (DNS)[edit]

Domain Name Server - translates domain names into IP addresses


If you have a modern mobile phone it is very unlikely that you will type in the number of your friends each time you want to call or text them. You are far more likely to use the address book, typing in their name, then letting the phone find the number. This is exactly the same principle behind a Domain Name Server. A DNS translates domain names meaningful to humans (such as www.google.com) into IP Addresses for the purpose of locating and addressing these devices worldwide.

Domain names are far easier for humans to remember than an IP address. To see a domain name in action you can use the ping command, where pinging google, uses the DNS to find the IP address you want to send a ping packet to:

# ping www.google.com
Pinging www.l.google.com [209.85.229.99] with 32 bytes of data:

Or on Linux try dig

$ dig www.google.com
-- Snip --
;; ANSWER SECTION:
www.google.com.         246     IN      A       173.194.34.144
www.google.com.         246     IN      A       173.194.34.148
www.google.com.         246     IN      A       173.194.34.145
www.google.com.         246     IN      A       173.194.34.146
www.google.com.         246     IN      A       173.194.34.147

Internet registries and Internet registrars[edit]

Internet Registrars - allow organisations and individuals to buy their own domain names


Internet registrars are responsible for allocating internet domains to anyone who wants one. If someone wanted to own their own website with a domain name, they would have to go to an internet registrar in order to buy the website name. These services typically require payment in order to maintain control over the name of the website. If you do not renew your website, then the internet registrars may sell it to other buyers.

Extension: Travel back in domain name time

In the early days of the World Wide Web, there were many instances of companies paying lots of money to buy their web domains from private individuals who were 'cyber-squatting'. Other organisations shared the same address and there were many legal fights and much cash exchanged to gain control of domain names. If you want to take a look at websites from the past take a look at http://web.archive.org and look at the following websites in the mid 1990s:

  • www.itv.com
  • www.michaeljackson.com

Internet Registries - hold domain names which are registered. They allow owners to link domain names to IP addresses. Well known domains usually have their own registry, such as .co, .com, .sch


You may ask why a company would want a domain to link to multiple IP addresses? If you think about a multinational company's website, if someone tries to load your homepage you don't necessarily want them to have to load this webpage from an server at an IP address on the other side of the world. So depending on their region you would point them at an IP address close to their current location. Typing in www.google.com in Europe and in Asia will send you to different IP addresses even though you have used the same domain name. Sometimes the content of the server won't be the same depending on your region.

Also big sites not only do geographic IP splitting but load balancing as well. This is when a domain name is undergoing heavy usage, and one particular IP address might be very busy, the domain name will then be pointed to other servers sitting at different IP addresses, balancing the 'load' of users accessing the site.

Internet Service Providers (ISP)[edit]

Internet Service Providers - companies which offer customers access to the internet


Getting a direct connection to the internet is quite costly involving specialist hardware. Most people and organisations pay ISPs to link them to the internet. Examples of Internet Service Providers are : AOL, BT, Sky, TalkTalk and Virgin Media.

Getting a direct connection to the internet isn't straightforward which is why most people and companies use ISPs
Exercise: ISP and DNS

Describe what a Domain Name Server does:

Answer :

Domain Name Servers translate domain names into IP addresses

Describe what ISP stands for and what is does:

Answer :

ISP = Internet Service Provider = companies which offer customers access to the internet

What is the difference between an Internet Registrar and an Internet Registry

Answer :

Internet Registrars allow people and companies to buy domain names.

Internet Registries keep track of who owns what domain name and what IP addresses each domain name links to

List 3 top level domain names:

Answer :

  • .gov .mil .com .org .xxx .net .uk .fr ...

Describe what the following domain names mean

  • www.itv.co.uk
  • en.wikibooks.org
  • policerecruitment.homeoffice.gov.uk

Answer :

  • www.itv.co.uk - a uk based, company called ITV, linking to the www host
  • en.wikibooks.org - an organisation called wikibooks, linking to the en host machine
  • policerecruitment.homeoffice.gov.uk - a uk based governmental site called homeoffice, linking to the policerecruitment host machine

List the three parts that make up a domain name:

Answer :

  1. host machine
  2. domain name
  3. top-level domain name

Give two reasons why a company might want to use a domain name instead of an IP Address for their website?

Answer :

  • Domain names are easier to remember
  • You can change the IP address without the user noticing
  • You can host several web sites on one IP address