What's my DNS number?
Interpreted as: Which DNS server should I use?
When setting up your computers to connect to the Internet it is important that you configure your computer to connect to the correct DNS server. Using the correct DNS server means that you will get the fastest and most reliable lookups for when you're visiting websites online.
While most routers and modems will automatically assign the correct IP address to your computer to use as a DNS server there is some cases in which you may need to configure this manually.
whatsmydns.net provides a DNS Server List that contains of many of the major ISP's throughout the world. When possible you should try and use the DNS server that is provided by your service provider in order to have the fastest possible DNS lookups.
It is also possible to use Google DNS as your DNS server if you want.