Monitor bandwidth with vnStat
Keeping track of bandwidth usage is a good thing for a variety of reasons, particularly in situations where bandwidth accounting is required, such as paying for Internet services that are dependent upon the amount of bandwidth used.
One great tool for Linux is vnStat, which is available from http://humdi.net/vnstat/. Some Linux distributions may bundle vnStat, but it is more likely that you will have to build it from source.
Begin by downloading the vnStat source tarball from the Web site and unarchiving it: