The Dreamcast is a home video game console by Sega, the first one introduced in the sixth generation of video game consoles. With the release of the Dreamcast in 1998 amid the dot-com bubble and mounting losses from the development and introduction of its new home console, Sega made a major gamble in attempting to take advantage of the growing public interest in the Internet by including online capabilities in the console as a selling point. As such, the Dreamcast was the first console to include a built-in modem for Internet support and online play. Sega would end up leaning heavily into the online capabilities to sell the Dreamcast as hype grew for Sony's then-upcoming competitor, the PlayStation 2, which also promised online gaming in addition to its DVD capabilities.

To create further incentive for use of the Dreamcast's online capabilities, Sega went beyond the scope of their prior online ventures and invested heavily in the development of unified online services for it, a concept that predated former partner Microsoft's Xbox Live service by a few years. Sega also predated Microsoft in pioneering the concept of downloadable content for games released on a console, though it was hampered by the small memory of the VMU. Despite the foresight Sega had in the emergence of broadband Internet access by making the modem modular and upgradeable with a broadband adapter, the services mainly supported dial-up Internet access throughout their lifetimes; only in Japan did broadband service arrive for the Dreamcast before Sega discontinued it in 2001, abandoning the console business altogether with its transition to third-party publishing. The services were gradually discontinued by Sega in the subsequent years; the last remaining service lingered on in Japan before it was shut down in 2007. In response, hobbyists have revived parts of the online services by creating private servers for a handful of games that had their official servers shut down.

Sega produced two networking accessories for the Dreamcast worldwide, one for dial-up connections and one for broadband connections. A third accessory, also intended for broadband connections, was only available in Japan. All adapters are visually similar to each other; however, the first accessory includes a telephone jack while the latter two accessories include an RJ45 jack for Ethernet instead. The adapters attach flush to an expansion port on the side of the Dreamcast. Sega also produced a keyboard and a computer mouse for easier navigation of the Internet on the Dreamcast.

The Modem Adapter is a dial-up modem that was included with most Dreamcast consoles sold worldwide, excluding Brazil and regions of Asia outside Japan. In Brazil, where the adapter was excluded due to the high price of the console there, it was sold separately for R$49.99 as the Dreamcast Link. European, Australian, and early Japanese models came with a 33.6 kbit/s modem, while North American and later Japanese models included a 56 kbit/s modem. However, with no region lock present, it is possible for models bundled with the 33.6 kbit/s modem to use the 56 kbit/s one instead for faster Internet access. To produce the modem, Sega partnered with Rockwell International through its semiconductor division, which was spun off as Conexant on January 4, 1999.

There are two models of the modem adapter, 670-14140A and 670-14140B. The "A" model can use power from the Dreamcast game console to allow it to operate without the need of power from the telephone line. The "B" model does not use power from the Dreamcast; thus, it is dependent on the power from the telephone line or a modem that runs power through the telephone line.^{[citation needed]}

There are multiple ways to create a server between a Dreamcast and a personal computer (PC) using the standard modem which allows the Dreamcast to share the PC's network connection.

The Broadband Adapter is a network adapter that was released as a separate accessory for the Dreamcast in Japan on July 15, 2000, retailing at ¥8,800; the United States followed suit on January 9, 2001, with a retail price of $59.95. CSI Co., Ltd. manufactured the adapter in Japan. The adapter was never bundled with any console; it was sold in Japan through CSI's website and cable Internet access providers, while the adapter was sold in the United States exclusively through Sega's online store from launch until March 2001, when it started appearing in retail stores. A European release for the adapter was planned for early 2001, but it ultimately did not materialize with the Dreamcast's discontinuation; despite that, use of an American (or Japanese) adapter on a PAL system is possible since the adapter is not region locked.

Only a few hundred thousand units of the Broadband Adapter were produced as worldwide broadband adoption was still poor at the time. While it was solely mass produced in white to match the console's default model color, a black model was produced on a limited build to order basis; orders were accepted throughout much of December 2001, with a minimum of 2,000 units produced from February to March 2002. Due to the adapter's launch late in the lifespan of the Dreamcast, only a handful of games supported the adapter as developers had to explicitly include support for the adapter in their games; the games that did support it were able to take advantage of the 10 and 100 Mbit speeds provided by the adapter. However, it also had an unintended consequence of much faster Dreamcast GD-ROM copying, leading to a dramatic increase in game piracy towards the end of the console's North American lifespan. CSI announced that it would discontinue production of the adapter on April 24, 2002, citing Sega's discontinuation of Dreamcast production and transition to third-party development.