File Extension R18
File extension R18 is most commonly associated with the initial volume from a spanned File extension RAR archive. Large RAR archive files can be split into multiple volumes to make it easier to store on removable media or to transfer the file over a network, and the first volume in the span will have the R00 file extension. Subsequent volumes of the archive will have the file extensions File extension R01, File extension R02 and so on up to a maximum File extension R99.
RAR is a proprietary file format which supports archiving and compression, and is similar in nature to File extension ZIP and File extension GZIP, though can offer an increased compression ratio depending on the types of file contained within the archive. The contents of a RAR archive may also be encrypted, and there is support for recovery should part of an archive become damaged through corruption or incomplete transmission across a network.
As archives are often used for backup purposes, it is conceivable that the file sizes are particularly large, even with the aid of compression. Depending on how the file needs to be stored, it may be necessary to split the archive into multiple sections to fit on media such as a CD or DVD-ROM. Additionally, if a large file is to be transferred over a local network or the Internet, it may be desirable to split the file into smaller sections to minimise the risk of the download failing or getting corrupted in transit. RAR archives can be split, with each volume in the span named with an incremental Rxx file extension starting with R00. Provided all the volumes of the span are present, the entire archive can be viewed by opening any of the individual segments.
R18 files can be opened using WinRAR, WinZip or Smith Micro StuffIt Expander, the latter of which is available on both Windows and Mac. However, as of WinRAR 3.0, spanned archives now use a different incremental file extension starting from .part000.rar.