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The 64DD (often referred to as the Nintendo 64DD) was an add-on to the Nintendo 64, released only in Japan after years of speculation and broken release dates. It had very little to no support from Nintendo, combined with its late release. Only 15,000 units were sold and only nine games were made for it, making it a commercial failure.
The whole purpose of the 64DD was to play online games. The 64DD plugged into the underside of the Nintendo 64, where the EXTension port is.