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Thank you for your question.
First off, you are correct, 6 hours downtime out of 54000 hours is 99.989% availability, and yes, 54000 hours is a little more than six years.
However, your assumption that you can annualize the downtime statistics as you describe is not quite correct. Six hours out of 54000 hours yields 99.989% availability, but if you annualize those numbers (dividing by six), you get one hour of downtime out of 9000 hours (slightly more than a year), which returns exactly the same 99.989% availability average per (slightly more than a) year.
If you know that in a particular year, you had less than an hour of downtime, then you'll get better availability statistics for that year, but for another year of the six, the statistics will be worse. If you know that yesterday, the PPTP line stayed up all day, that's 100% availability for the day. Daily availability statistics are rarely useful, though.
To calculate availability statistics, you count up all the time you were down (6 hours), divide that into the total amount of time over which you have collected the statistics (54000 hours). You should then have a very small number (0.00011). Subtract that small number from 1 (0.99989). Multiply that by 100 to get an availability percentage (99.989%).
Hope this helps.
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