This article is contributed by Sarah Scrafford, who regularly writes on the topic of web design degrees. She invites your questions, comments and freelancing job inquiries at her email address: sarah.scrafford25@gmail.com.  

We've all come across it at some time or the other and accepted it at face value, that is, at least until the actual reality proves to be otherwise. Web hosts who claim an uptime of 99.99 percent are putting up those numbers on their advertisements because they know how important uptime is to a vast majority of sites that jostle for space and attention on the World Wide Web. No one goes so far as to explain how they came up with all those nines on the number, but they do know that if they end up keeping your site down for too long or one too many times, they'll end up losing an existing customer and many more potential ones.

And that's because it's an age where uptime has taken on more meaning than ever before - with Web 2.0 and social networking sites, blogs and the like taking up oodles of storage space and lots of bandwidth, they cannot afford more than planned, or at the very worst, semi-planned downtime. While the first refers to times your host plans maintenance tasks, periods that are announced well in advance, the second denotes times that important security updates and patches have to be installed, with the time period being announced as soon as possible but not as soon as clients would normally like.

But the third and most unsettling of downtimes is that which happens without the hint of an announcement - unless you have the resources to monitor your site at all times, you may not even be aware of such outages until some well-meaning friend brings it to your attention. This happens for various reasons - traffic bursts on a few sites on shared servers can cause other sites on the same server to shut down for lack of bandwidth; hardware or software malfunctions on the server are likely to cause outages; and the worst, malicious software could be used to target your site and prevent others from accessing it.

The last kind of downtime is the one that Web 2.0 sites are keen to avoid - they maintain a large number of servers and databases, and a denial of service attack is the last thing they need. The downside of popularity, if you can call it that, is to have to be constantly on the lookout for anything that could jeopardize their site's uptime, the feature that all websites want a hundred percent of.