On the page entitled What is propagation? we discovered that the DNS is like a telephone directory. It contains the common name of a website (its URL) and its address (IP address). When a URL is entered into a browser, the browser performs a DNS lookup to convert that URL into an IP address and then sends a web page request to that IP address.
There are potentially millions of DNS lookups being performed around the world in any one minute, so for there just to be one DNS server is impractical – it would never be able to cope with all the lookups being requested of it. As a result, there are lots of DNS servers around the world.
When your browser makes that DNS lookup it uses a DNS server local to it, as determined by your Internet Service Provider (ISP).
When a new website is created, its URL and IP address will need to be added to the DNS. It’ll be added to a DNS server by the company that registers the URL for on behalf of web site’s owner. However, in that instant when the domain name is added to the first DNS server, none of the other DNS servers around the world will know that the web site exists. This is because that first DNS server won’t have shared the new web site’s details with any other DNS server yet.
Over the next 48-72 hours, the DNS around the world will share all their new websites, changes and deletions. This sharing of DNS information is known as DNS propagation. The network of DNS servers around the world consists of large servers run by large network providers and smaller servers run by other organisations. Generally, the DNS information will be propagated between the larger servers first and the smaller servers will obtain the changes from these larger servers. There is some order and hierarchy to the potential chaos that could exist in DNS propagation.
For popular websites, DNS results may be cached for people in different parts of the world using many different recursive DNS resolvers. If you have recently made changes to your configuration, and the TTL has not yet expired, then some people may be receiving out of date results which could mean that they see an older version of your website.