A web hosting service is a type of Internet hosting service that allows individuals and organizations to make their own website accessible via the World Wide Web. Web hosts are companies that provide space on a server they own or lease for use by their clients as well as providing Internet connectivity, typically in a data center. Web hosts can also provide data center space and connectivity to the Internet for servers they do not own to be located in their data center, called colocation or Housing as it is commonly called in Latin America or France.
The scope of web hosting services varies greatly. The most basic is web page and small-scale file hosting, where files can be uploaded via File Transfer Protocol (FTP) or a Web interface. The files are usually delivered to the Web “as is” or with little processing.[1] Many Internet service providers (ISPs) offer this service free to their subscribers. People can also obtain Web page hosting from other, alternative service providers. Personal web site hosting is typically free, advertisement-sponsored, or inexpensive. Business web site hosting often has a higher expense.
Single page hosting is generally sufficient only for personal web pages. A complex site calls for a more comprehensive package that provides database support and application development platforms (e.g. PHP, Java, Ruby on Rails, ColdFusion, and ASP.NET). These facilities allow the customers to write or install scripts for applications like forums and content management. For e-commerce, SSL is also highly recommended.
The host may also provide an interface or control panel for managing the Web server and installing scripts as well as other modules and service applications like e-mail. Some hosts specialize in certain software or services (e.g. e-commerce). They are commonly used by larger companies to outsource network infrastructure to a hosting company.
The availability of a website is measured by the percentage of a year in which the website is publically accessible and reachable via the internet. This is different than measuring the uptime of a system, as uptime refers to the system itself being online, however does not take into account being able to reach it – as in the event of a network outage.
The formula to determine a system’s availability is relatively easy: Total time = 365 days per year * 24 hours per day * 60 minutes per hour = 525,600 minutes per year. To calculate how many minutes of downtime your system may experience per year, you can take your uptime guarantee and multiply it by total time in a year.
In this example I’ll use 99.99%: (1 – .9999) * 525,600 = allowable minutes down per year.
The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable per year, month, or week.
| Availability % | Downtime per year | Downtime per month* | Downtime per week |
|---|---|---|---|
| 90% (“one nine”) | 36.5 days | 72 hours | 16.8 hours |
| 95% | 18.25 days | 36 hours | 8.4 hours |
| 97% | 10.96 days | 21.6 hours | 5.04 hours |
| 98% | 7.30 days | 14.4 hours | 3.36 hours |
| 99% (“two nines”) | 3.65 days | 7.20 hours | 1.68 hours |
| 99.5% | 1.83 days | 3.60 hours | 50.4 minutes |
| 99.8% | 17.52 hours | 86.23 minutes | 20.16 minutes |
| 99.9% (“three nines”) | 8.76 hours | 43.2 minutes | 10.1 minutes |
| 99.95% | 4.38 hours | 21.56 minutes | 5.04 minutes |
| 99.99% (“four nines”) | 52.56 minutes | 4.32 minutes | 1.01 minutes |
| 99.999% (“five nines”) | 5.26 minutes | 25.9 seconds | 6.05 seconds |
| 99.9999% (“six nines”) | 31.5 seconds | 2.59 seconds | 0.605 seconds |
* For monthly calculations, a 30-day month is used
A hosting provider’s SLAs may include a certain amount of scheduled downtime per year so that they can perform maintenance on the systems. This scheduled downtime is often excluded from the SLA timeframe, and needs to be subtracted from the Total Time when availability is calculated. Depending on the verbiage of an SLA, if the availability of a system drops below that in the signed SLA, a hosting provider often will provide a partial refund for time lost.
The percent uptime advertised by a web host is often a bad metric for determining a hosts quality. With this metric, a scheduled downtime from 2AM-3AM will be counted as just as bad as an unplanned downtime from 5PM-6PM.
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