The first International Docking Adapter installed on the ISS was IDA-2.
IDA-2 arrived aboard SpaceX’s CRS-9 Dragon cargo mission in July 2016. Astronauts Jeff Williams and Kate Rubins installed it during a spacewalk on 19 August 2016, attaching it to Pressurized Mating Adapter-2 at the forward end of the Harmony module, also known as Node 2.
The adapter converts the station’s older APAS-95 interface into the NASA Docking System interface used by spacecraft built to the International Docking System Standard. It allows compatible vehicles to approach and dock autonomously, transferring crew, cargo, power and data through the connection.
The numbering can be misleading. IDA-1 was intended to be the first installed, but it was destroyed when SpaceX’s CRS-7 launch failed on 28 June 2015. IDA-3 later arrived on CRS-18 and was installed on PMA-3 in 2019, giving the station a second modern commercial-crew docking port.