Something about Domain Name Servers

Domain Name servers are responsible for resolving the web address that you type in on the address bar of your web browser into  machine understandable IP addresses to locate the servers…

Almost all the internet service providers have their own dns servers for  this purpose… It is not strict rule that you have to use this dns servers for address resolution always.. some other public and free dns servers are available today.
Some of these kind of dns servers provides security against vulnerabiliteis in the internet, some are very much faster than the dns servers of your ISP..
The ip of  best public dns servers available are

google public dns
8.8.8.8
8.8.4.4

opendns
208.67.220.220
208.67.222.222

dns advantage
156.154.70.1
156.154.71.1

others
209.244.0.3
209.244.0.4

How to find out the best dns server for your system?

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This cheking simply consists of pinging from your system to all the nameservers one after another and to see the one that is having minimum average time
to do this, take your terminal and type in as follows

1: $ ping -c 5 8.8.8.8

2: Checkout the average time needed for the process.
Its shown in    the terminal. Replace the address
8.8.8.8 with all other  dns addresses

How to change the dns server…
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1. take terminal from Applications -> accessories -> terminal

2. Backup the resolv.conf file in the etc folder of the filesystem

                $ sudo cp /etc/reasolv.conf   /etc/resolv.conf.bak

3. Edit the back up file to change the dns address

                $ sudo gedit /etc/resolv.conf.bak

4. replace the current domain address specified there with the

dns  server address that gave you the minimum average time.

Enter the other address below that in the same format if you

wish..(Network manager opt for secondary address if the

connexion fails)

5. That’s all about making the changes… The data that u

have written in the back up file will long till u delete that file

manually.

6. Copy back the content of the backup file (resol.conf.bak) to the original 

resolv.conf after you connected to the internet whenever you want to change 

the address (before

starting  browsing)


update

You can use the network manager to automatically change the dns address. Assign the dns server address in the IPv4 settings tab as shown in the figure.