DNS Lookups: How do they work?

A fair few people know what DNS lookups are, Even without having to work in the IT industry. It basically converts the pretty, familiar and easy to understand (for us) “www.domain.com” into an IP Address, which computers can understand and direct traffic to.

The Image below is from Verisign, and to make the understanding a bit easier, it ignores the process of caching.

DNS lookups

Image source: Verisign Domain Name Industry Brief, June 2007 (PDF), last page.

First your computer queries the name server (DNS server) it is set up to use. This is the recursive name server shown above.

The name server doesn’t know the IP address for www.abc.com, so it will start the following chain of queries before it can report back the IP address to your computer (the numbers below correspond to the numbers in the image).

  1. Query the Internet root servers to get the name servers for the .com TLD.
  2. Query the .com TLD name servers to get the authoritative name servers for abc.com.
  3. Query the authoritative name servers for abc.com to finally get the IP address for the host www.abc.com, then return that IP address to your computer.
  4. Done! Now that your computer has the IP address for www.abc.com, it can access that host.

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