Domain Name System (or Service) is how the Internet can properly route your request for a website. The link explains DNS completely but briefly: www.microsoft.com is actually found on the web via an IP address, not the familiar domain name of microsoft.com. So when you type www.microsoft.com in the addressbar, your request is first intercepted by a DNS server. The DNS server looks up the IP address of your request then forwards you to the desired website.
Hijacked DNS settings
We have seen where some computer's DNS setting were hijacked, so instead of obtaining the DNS addresses from the router or provider, there were odd numbers in the settings (see below). Those hijacked DNS settings meant everything done on the Internet was FIRST routed through the hijacker's server, so they were seeing (probably recording) everything submitted.
If you are getting redirected, i.e., want to go to one website but end up at another site, or you suddenly have all kinds of ads popping up, or get a lot of page not found (404) errors or have any other strange issues with the Internet, get your computer checked! See Hijacked! for more.
By default, the DNS addresses used by your router and browser are assigned by your Internet Service Provider (ISP) and automatically obtained. But like any other computer, DNS servers can go down (typically it's only for a moment). Sometimes DNS servers are removed from service then replaced with different DNS servers. If the DNS servers change (and you will never know when they do), generally powering off and up your mode and router will pickup those changes.
Check the current DNS assignment:
you may consider using Google's DNS addresses of: 184.108.40.206 and 220.127.116.11 instead.
If you have a router, that is where this change should be done. See Routers
If your computer connects directly to the DSL modem, you can over-ride the ISP assigned DNS addresses in the network connection properties...
2) Click the Properties button (not shown).
3) Then from the list, locate and select Internet Protocol TCP/IP Version 4 and click Properties.
4) Leave the top section as is, changing only the lower DNS
Note: If you want to, you could have the ISP DNS addresses under Advanced for a total of 4 DNS servers, but that's really not necessary.