Overview

DNS can map a server-hostname an addressable machine (by IP address) on the Internet.
A server-hostname may be comprised of a domain name + optional subdomain. (e.g. example.com or www.example.com).
In order to do this, the DNS-Resolver of a computer/client may issue  multiple queries, recursively, until the machine address is resolved.

Example (simplified)

• A User would like to use his web-browser (Firefox) to visit wikipedia.com
• For this the name wikipedia.com needs to be resolved to an IP-Address.
• A Query/request gets sent to the "resolver"/dns-server configured in the Network Settings.
• The dns-server asks the root (.) nameservers where com. is and gets an answer (responsible TLD-Servers).
• The dns-server asks the TLD-Servers where wikipedia.com. is and gets an answer. (responsible nameservers from a web-host)
• The dns-server asks the webhost's nameservers what example.com. is. The answer is an IP-Address of a web-server.
• Now the Web-Browser can make a request to the correct webserver and display wikipedia.com to his user.

Cached Queries

As a lot of requests/queries are required to resolve a single name to an IP-address.

One of the possible optimizations is for the local resolvers/dns-servers to “cache” results it retrieved before and store them for a specified amount of time.
(This is called TTL…Time To Live)

Important: Due to this mechanism DNS-Updates will take some time to “propagate”. Changes may take up to 24 hours to take full effect.