If memory serves, the first (at least publicly disclosed) interface was the "Dreamcast Debug Handler", created by a member of Hitmen. This worked by adding a br

If memory serves, the first (at least publicly disclosed) interface was the "Dreamcast Debug Handler", created by a member of Hitmen. This worked by adding a breakout connector to the expansion port terminator (which came with some Asian Dreamcast systems instead of a modem), then connecting that to some homebrew hardware to adapt the bus to a parallel port. Another alternative that was reasonably-priced for a short while was the Japan-exclusive "LAN Adapter", which was in lower demand because it was only officially supported by the Dreamcast web browser app and was only 10Mbps instead of 10/100.