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99.9999% Uptime

Some hosting providers, as with 24/7 support, like to rattle off the 99.9999% uptime guarantee, or some version of it. They may even be so bold as to offer a 100% uptime guarantee! The latter is not even worth addressing; like claims of unlimited this and that, a 100% uptime guarantee isn't worth the pixels used to present the number on your screen. Absolutely nothing is 100% fail-proof, and providers that offer such guarantees carefully hide, in the fine print, the fact that the guarantee applies to the network, not the server. With multiple network connections being standard, providers offering such a wonderful guarantee are not doing anything special.

Besides, as with all statistics, this too should be taken with a grain of salt. You should ask yourself (and your [potential] hosting provider), "99.99% of what time frame?" Yes, there's a pretty good chance that you'll have 100% uptime between now and this time tomorrow but, statistically-speaking, the probability drops off as the length of time increases to a week, a month, a year, and beyond. For a service to be available, the application (program), the operating system, the server (hardware) and the network all have to be up; if one of them is down, even if the other three are up, your overall 100% uptime just went out the window.

Let's take a look at exactly what the numbers mean. Assuming there are an average of 365.25 days in a year, this means that there are an average of about 730 hours or 43 830 minutes or 2 629 800 seconds in a month. Here is the amount of possible downtime for each claim, with the easier-to-absorb numbers shaded:

UptimePossible Monthly Downtime
100     0    0 0
 99.9999     2.6298    0.0438 0.0007
 99.999    26.298    0.438 0.0073
 99.99   262.98    4.38 0.0731
 99.9 2 629.8   43.8 0.7305
 9926 298  438 7.305
 9852 596  87714.61
 9778 8941 31521.915

As you can see, if your host was crazy enough to offer only a 98% uptime guarantee, they would be telling you that you could count on your server being down for almost two entire work days every month, and even 99% isn't much of a better situation. Would that be acceptable to you? Probably not, and you're not going to find any hosts offering a guarantee like that, and any hosts who suffer only 98% uptime aren't going to be in business for very long unless they fix whatever problem is causing them to be down so much. Additionally, as in most industries, service agreements limit damages to what you have actually paid (i.e., consequential damages are excluded), so if your site is down for a day and you lose $1000 worth of business as a result, your host is only going to credit you with 1/30 of your $4.95 -- i.e., about 17¢.

The point here is that a 99% uptime guarantee (or some variation of it) is not much of a guarantee and there's nothing special about it; it's a given. Telephone companies don't even talk about uptime (although, generally speaking their goal is apparently 99.999% uptime), as everyone assumes that they will be up all the time unless there is some major catastrophe.

Relevant links:
NinerNet Communications™: Web and email hosting, domain registration, SSL certificates.