An ISP is drafting a Service Level Agreement with specific downtime limits. What percentage should be used to meet these metrics?

To determine the appropriate percentage for downtime limits in a Service Level Agreement (SLA) drafted by an Internet Service Provider (ISP), it’s important to understand how these percentages correspond to actual uptime in terms of hours or minutes over the course of a year.

When considering the options provided, a percentage of 99.999%, often referred to as "five nines," is a common standard for high availability. With five nines, the total allowable downtime is reduced significantly compared to lower percentages. Specifically, 99.999% availability translates to roughly 5.26 minutes of downtime per year, which is exceptionally reliable for critical services. This level of availability is often demanded in sectors like finance and healthcare, where even minimal downtime can lead to significant consequences.

In comparison, 99.9% (three nines) allows for about 8.76 hours of downtime annually, which is considerably more than what many businesses want. Similarly, 99.99% (four nines) permits around 52.56 minutes of downtime per year, still a higher threshold than five nines. The highest option, 99.9999% (six nines), drastically reduces downtime to approximately 31.5 seconds per year and is rarely