Welcome to the ITFreeTraining course on DNS or the “Domain Name System”. This course
is available free of charge on YouTube and will take you through everything that you
need to know about DNS. If you find that it does not let us know on our web site what
is missing and we will look into adding it. As DNS changes so will these videos, keeping
you up to date on the latest DNS features. Now to get started, what is DNS?
DNS, or the “Domain Name System”, is a system primarily designed to make computers
easier to use for humans. In computing, every computer has an IP address which essentially
is just a series of numbers. DNS is a system which maps a name to an IP address, just like
a phonebook maps people’s names to their phone numbers. Just like phone numbers,
you can probably remember a couple but it is difficult to remember a lot of them. In
comparssion as a person's name is easier to remember than their phone number, computer names are
generally easier to remember than IP addresses.
Before DNS was developed, a file was created called the hosts file. This file was stored
on the local computer and is still present in modern operating systems. The hosts file,
as shown, allows you to map an IP address to a more human friendly name. In some rare
circumstances you may use the hosts file, however in most case you will want to use
DNS where possible since it makes administration a lot easier.
Before DNS, the hosts file had to be copied between computers when changes were made.
With a few computers this is not too difficult, but when the internet started to expand and
more computers were added, this became harder and harder. Hosts files also became larger
and larger and thus there were more changes - making it hard to ensure clients always
had the most up to date copy of the hosts file.
DNS was developed back in 1983. DNS is a hierarchy-based distributed database
that allows computers to find computers and services on the internet. It is considered
a hierarchy rather than flat because you can have multiple computers with the same name.
For example, you could have two computers with the hostname DC1. DNS knows they are
different hosts because the full name, otherwise known as the fully qualified domain name,
is different. In this case, one is for ITFreeTraining.com and the other is for HighCostTraining.com.
Older systems like NetBios, would only allow one computer on the network with the same
hostname. With DNS, as long as the fully qualified domain name is different, you can have as
many hostnames as you want with the same hostname.
In order to a have a centrally-controlled but distributed namespace, the DNS namespace
is broken up and put on different servers all over the world. If you consider it like
a piece of pie, you can see that different servers hold different parts of the namespace.
In a later video I will look closer at how the namespace works, but you can see here
that there are servers that hold only part of the namespace like “com”.
For individual name spaces there are servers that hold this namespace. For example, there
is a DNS server that holds the Microsoft or ITFreeTraining part of the namespace. Dividing
the namespace up like this allows central control, but at the same time allows individual
administrators to make their own changes as required.
This has been just a short introduction to DNS. In the upcoming videos I will be look
at all the different aspects of DNS. If we don’t cover the DNS topic that you are interested
in, please feel free to go to the web page and let us know. As DNS changes, we will add
and remove videos as required -keeping the course up to date, not just for this course
but other free courses we offer. Thanks for watching and I hope to see you in the next
video.