The domain name system (DNS)

Skills: none

IT concepts: domain, domain name, host, IP address, domain registrar

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The domain name system (DNS) in context

• Internet concepts– Applications– Technology (TCP/IP)– Implications for

• Individuals• Organizations• Society

• Internet skills– Application development– Content creation

• Text• Images• Audio• Video

Domain names

We refer to computers on the Internet (Internet hosts), by names like:

bpastudio.csudh.edu

These are called domain names or, if you want to be really geeky, “fully qualified domain names.”

The key point is that the name identifies a particular computer – no two hosts have the same domain name.

The parts of a domain name

The particular host is called bpastudio

The organization that controls it is called csudh

This host is at an educational organization

bpastudio.csudh.edu

A domain is a group of related hosts.

How are these hosts related?

• bpastudio.csudh.edu• www.ucla.edu• sws.csudh.edu• chemistry.stanford.edu

They all belong to educational institutions.

They are members of the edu top-level domain (TLD).

How are these hosts related?

• bpastudio.csudh.edu

• www.csudh.edu

• sws.csudh.edu

• chemistry.csudh.edu

They all belong to the same organization, CSUDH.

They are members of the csudh.edu second-level domain.

A unique host

bpastudio.csudh.edu

This is the domain name of a unique host. It is a member of the csudh.edu domain, but it is the only one with the name bpastudio.

It is like a family – all the brothers and sisters have the same last name, but it would be confusing if two of them also had the same first name.

There are a limited number of top-level domains (TLDs), including:

• .edu, educational• .com, commercial• .gov, government• .org, non profit• .net, networking organizations

These are called “generic” TLDs.

There are also country code top-level domain names for every nation, like:

• .us, United States• .mx, Mexico• .cl, Chile• .uk, United Kingdom• .tv, Tuvalu

These are called “country code” top-level domains (ccTLDs).

The organization or host do not necessarily have to be in the country to register the name. For example, .tv is popular everywhere.

Some TLDs use a four-level hierarchy

• http://www.google.co.uk/

• http://www.unam.edu.mx/

• http://www.ox.ac.uk/

• http://www.google.com.mx/

Registrars sell domain names

• For most generic TLDs there are many registrars and a competitive registration market place: – http://www.icann.org/en/registrars/accredited-l

ist.html• The registrar for .edu is a professional society

called Educause:– http://net.educause.edu/edudomain/

• List of organizations responsible for registrars in the country code domains:– http://www.iana.org/domains/root/db/#

Regstering a domain – check for availability

Larrypress.com was taken, but these are available and for sale

Getting a name and IP address at CSUDH

• A computer center staff person assigns them.

• No two IP addresses or domain names can be the same.

• The staff person updates the table of domain names and IP addresses stored on the CSUDH domain name server.

Some hosts in the csudh.edu domain

Domain name IP address

sws.csudh.edu 155.135.1.163

ctl.csudh.edu 155.135.1.166

email.csudh.edu 155.135.1.180

ns1.csudh.edu 155.135.55.2

vaquero.csudh.edu 155.135.55.6

toro3.csudh.edu 155.135.55.8

music1.csudh.edu 155.135.55.11

home.muphiepsilon.org 155.135.55.13

library.csudh.edu 155.135.55.17

We are letting muphiepsilon.org use a computer on our campus.

Some hosts in the csudh.edu domain

Domain name IP address

sws.csudh.edu 155.135.1.163

ctl.csudh.edu 155.135.1.166

email.csudh.edu 155.135.1.180

ns1.csudh.edu 155.135.55.2

vaquero.csudh.edu 155.135.55.6

toro3.csudh.edu 155.135.55.8

music1.csudh.edu 155.135.55.11

home.muphiepsilon.org 155.135.55.13

library.csudh.edu 155.135.55.17

Every domain has a domain name server (DNS). Our DNS is ns1.csudh.edu. It does the IP address lookups for TCP..

DNS search

• If your DNS knows the IP address it returns it.• Otherwise it will contact another DNS to get the

result.• Etc. until it is found or determined that it does

not exist, in which case an error message is returned.

Food for thought

• Is the <your name>.com domain name available?

• When you try to go to http://www.google.xom, you get an error message, but http://www.gooogle.com takes you to Google. Why?

• http://wwww.google.com also returns an error message. Why?