DNS is one of the key parts of what makes the internet world go round. Think of it of a big address book of website servers, mail servers and similar. Without a working DNS, a lot of the internet would simply stop working too.
If you know  how DNS works, then you will already be aware that when a website changes where it is hosted – or perhaps a change of where your email is hosted, then the DNS pointers  (A records and MX records will need to be updated). All around the world – suddenly everyone’s copy of the DNS information held on lots of separate servers for reliability and resilience is now out of date. If you ever change hosting provider and your IT company or Web design people explain that it may take between a few hours to a couple of days for everyone round the world to see your new website – it is the DNS changes or DNS propagation (technospeak) that is happening, and when that is complete (when all or most Internet Service Providers or Public DNS providers have up to date records) – then your new website or email host should be rocking and rolling.

The big question for many people is – how will we know when the changes have happened. Well, given that there are many hundreds of thousands of DNS servers in the world – we cannot say with absolute certainty – but there are certain hierarchy (google Root DNS servers)  and timing (google TTL) arrangements that come in to play. It is possible to manually query a DNS address book to check whether it still has the old or new information – but my tip today is to give you a website that will help you query this more easily:

http://www.dnsunlimited.com/propagation_check

Pop in the name of your domain and the type of DNS record that you want to check – and it will quickly go and make some DNS enquiries for you at different places round the world. This should help give you some idea as to whether your changes have started or are in progress, or have completed.