The 64DD came with a modem cart. It fits into the cart slot, right where a game would go. There is a green light on the front to show when the modem is active.
Inside the modem is an ASIC and a ROM. The ROM had to be dumped the "hard way", by removing the ROM chip from the board and connecting it to a reader. The ROM is 32mbits and has a base address of 0x18000000, with a mirror at 0x18400000. It begins with a standard 64-byte cart header, followed by three ELF files at 0x40, 0x10000, and 0x20000. Each ELF appears to implement a softmodem. It is not clear why there are three different versions.
The Randnet Disk doesn't recognize the modem if its ROM is physically removed from the board.
Both the Randnet Disk and the Communication Kit dial a built-in phone number that cannot be changed: 03-3568-5050. This number is no longer in service.
Trivia: this is one of the few N64 accessories that uses triwing screws.