What is the Domain Name System (DNS)?

When you put a website on the Internet, it will be reachable via the numerical IP address of the web server hosting it.
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208.65.153.238 is an IP address
IP addresses are, however, not easy to remember for humans. Using them to identify online resources is also not practical as services on the Internet occasionally have to move to a new IP address (if they change service providers, for example).
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As the use of IP addresses for websites is neither practical nor user friendly, “domain names” such as youtube.com were created .

The global Domain Name System works a little like a phonebook for the Internet.
If you know the domain name of the website you want to visit, the Domain Name System is used – invisibly and automatically – to find the corresponding IP address of the web server where the website can be found. So, when you type xyz.com the computer identifies this as being 208.65.153.238 and sends a request specifically for the website to retrieve it.
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                      The system for looking up a domain name works on the basis of a hierarchy. When you type http://youtube.com,  your computer first connects with a server to ask for the address. The default DNS server is usually run by your Internet provider, but it is possible to use a different one.
If somebody has recently accessed http://youtube.com
the DNS server will “remember” the details and provide you with the correct IP address.
If not, it will refer the query to a higher level of authority, where the same process is followed. At the highest level of authority are 13 “root servers” that ultimately collect together DNS servers.
             The 13 root servers are very robust and have huge capacity. They have so much capacity that they continued to work efficiently even when under major attacks (so-called ‘distributed denial of service’ or DDOS attacks)

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