The Internet was a worldwide computer network on Earth. It was developed in the late 1960s, originally for military use, but rose to public prominence by the late 20th and early 21st century as a dominant source of communication, information and commerce.
UNIT maintained a website with restricted access sections that allowed remote launch of non-nuclear UK missiles. (TV: World War Three)
search-wise.net was a search engine operational in 2005. Rose Tyler used it to look for information about the Doctor, which led her to whoisdoctorwho.co.uk. (TV: Rose)
Mickey tracked information about the Doctor between 2005 and 2006. (TV: Aliens of London) The Ninth Doctor, realising this would make him noticed, devised a computer virus, designed to delete all references to the Doctor from the Internet, and asked Mickey to run the program. (TV: World War Three)
In 2006, Mr Cleaver put up his concerns about the safety of the Blaidd Drwg nuclear power plant on the Internet. Cathy Salt discovered these results. (TV: Boom Town)
In 2008, the Eleventh Doctor used a webchat feature to communicate with a number of world organisations using Jeff Angelo's laptop; he later advised Jeff to delete his Internet history. (TV: The Eleventh Hour) This incarnation of the Doctor had a particular disdain for the popular messaging service Twitter. (TV: The Power of Three)
In 2012, Henry van Statten was the self-proclaimed owner of the Internet. At that time, connections throughout the Internet were fast enough for a Dalek to download its entire contents within seconds. (TV: Dalek)
In 2025, the Fourth Doctor described the Internet to his companion Leela as "a 21st century human obsession." By this time, there was holographic Internet access. (AUDIO: Energy of the Daleks)
The Internet, in some form, was still in use on Earth in the very late 21st century, as webpages relating to the Bowie Base One crew and Adelaide Brooke's granddaughter existed. (TV: The Waters of Mars)
Donna Noble, desiring to go shopping, once asked the Tenth Doctor if the TARDIS had Broadband (i.e. Internet access). (COMIC: The Greatest Mall in the Universe)
Behind the scenes
The Internet has played a major role in promoting both the classic and revival Doctor Who series since the late 1990s. The BBC's website offered original story content (Scream of the Shalka, Real Time, Death Comes to Time, etc.) for the Internet, with exclusive-to-Internet short stories also uploaded from time to time. Official Twitter accounts related to the franchise are also maintained. Both Torchwood and The Sarah Jane Adventures have had official websites and exclusive online content uploaded over the years as well.
When the series returned to TV in 2005, the BBC launched a "viral marketing" campaign by creating faux websites based upon characters and organisations featured in the series, such as the aforementioned UNIT site, which fans could visit. The primary viral site during the 2005 season was Mickey's Defending the Earth! site, which was updated each week with teasers or background information (all presented in an in-universe style) relating to that week's episode. The site included a message board that occasionally featured messages from recognisable characters, such as Sarah Jane Smith a year before she returned to the series in School Reunion.
The search-wise.net website used by Rose actually exists. It is a "dummy site" for use by movie and TV producers in lieu of using recognisable sites like Google, although, in Steven Moffat-produced scripts such as The Girl Who Waited and The Power of Three, references to real-life sites like Twitter have been increasingly included.
A large number of books and audio dramas are currently available for download via the Internet, with a growing number of official and unofficial books also being made available only as e-books and thus available only via the Internet.