The .r21 file extension is part of a . When a file is too large to be easily shared or stored on a single disk, developers use tools like WinRAR to "split" it into multiple volumes.
: WinRAR (the original developer of the format) or 7-Zip (a popular free, open-source alternative). Download r21 rar
: The number "21" indicates that this is the 21st part of a compressed archive. Download r21 rar