DNS servers are used by internet providers to navigate the web - listing web addresses that end with .co.uk, for example, and translating them into their numerical internet protocol (IP) address.
A failure in this system can leave users unable to access some websites despite still having an internet connection.
Internet users in London, as well as the rest of the UK, were hit with the blackout.
According to monitoring website Down Detector fixed internet being the most commonly reported problem area.
