PAR2 files contain error-correction data that can be used to fix files that become corrupted during the upload/download process on Usenet where there is no built-in error correction. PAR2 files work on both Windows and Mac, and can be used to protect any kind of file.
PAR2 files don’t have a separate life of their own. You don’t decompress them like you do RAR files. PAR’s only exist to fix other files. Sometimes in a newsgroup you will see a set of PAR’s posted with no other files. The poster did that as a favor to somebody who needed to fix a file that he had downloaded previously. So, you don’t need to download PAR files unless you need them to fix a damaged file in your possession. Of course, it is not always obvious if a file is damaged. For example, there could be small flaws in a video that you can’t find unless you watch the whole thing very closely. It is not a bad idea to download the PAR’s to make sure that your files are OK.
PAR2 files are able to magically fix, and even re-create missing files. Suppose you download 9 of the 10 RAR’s from a binary newsgroup that contain a video file, and you can’t find the 10th. If the poster included some PAR files, you can download them and actually re-create the missing file! If the poster did not include any PARs, as newbie posters often do, you could post a message in the group and ask somebody who was able to get all the files to make, and post some PARs for you. Anybody can make PARs as long as they have a good copy of the files in question.
There are two generations of PAR technology in use: PAR and PAR2. PAR filenames look like this:
somefile.p01
somefile.p02
somefile.p03
PAR2 filenames look like this:
somefile.par2
somefile.vol000+01.PAR2
somefile.vol001+02.PAR2
somefile.vol007+05.PAR2
To use PAR files, open the first PAR or PAR2 with the free PAR decoder in SuperNZB. SuperNZB will use the PARs to examine the target files (RAR files, AVI files, etc.) and fix them if needed.
Both PAR and PAR2 do the same thing, but PAR2 is much better. Look at the PAR2 filenames above. The ‘+01’, ‘+02’, and ‘+05’ indicate how many ‘blocks’ each file has, and that can save you download time. To do that, you would first download the associated files whether they are RARs, AVIs, etc. Then you would download the file that ends with ‘.par2.’ Run that file through your un-PARing program, and it will tell you how many more blocks you need, if any. If you needed seven blocks, then you could download the second and third PAR files. If you only needed two blocks, then you could just download the second file.
PAR files cannot contain viruses, so there is no risk in using them.
PAR uses the Reed-Solomon algorithm. Dr. Dobb’s Journal has a good technical explanation here.