DNS (Domain Name System) is an Internet service that converts user-friendly domain names into the numerical Internet protocol (IP) addresses that computers use to talk to each other. For example, when you go to our web site, the domain name is "isitdownrightnow.com", which is all nice and easy to remember (a bit long though), but it doesn't help your computer at all. Your machine is set up to use something called an IP address, which is a bunch of numbers instead of the letters we are used to seeing. You can try to copy and paste our ip address 18.104.22.168 in your browser and it will open our site as well.
Why use other DNS Services ?
DNS servers are operated by your Internet service provider (ISP) and are included in your computer's network configuration. If you can access a website from a cellphone on 3G, neighbours computer or office, your ISP could be the source of the problem. If a website change their ip address your ISP should also update their DNS. This update can be slow on some providers. Also if the DNS server of your ISP is slow, the time it takes to resolve the web address adds up to the overall loading time of the website.
How to configure your PC to use Alternative DNS Services
Instead of using your ISP DNS service, you can try free DNS services such as OpenDNS or Google Public DNS to solve ISP specific connection problems. Choose your operating system below for OS specific instructions.