• Web server got hacked via exploit 
• Denial of service attack
• Internet connection went down
• Hardware failure e.g. hard drives, memery, processor etc.
• Software failure e.g. operating system, server software 
• Virus infection
• Maintenance on web server that require reboot
• Electricity outage (power failure)
• Fire in building
• Natural disasters e.g. earthquacke, tornado etc.
• You didn't pay your monthly hosting service fee

I have not found one web hosting company that guarantee 100% uptime or availability. Even top companies like Google don't guarantee 100% uptime for their hosted services.

It would be great to get 100% unaffected service from the web hosting companies, but unfortunately this is not possible. Lots of web hosting companies struggle with this factor and big companies spend millions to keep their server online, but even then they can only promise 99.99% up-time. 

The big web hosting companies will have disaster management and systems in place that will help with keeping their web servers online. They will have more than one power source in case one drop and backup power generators. They will also have more than one internet connectivity line in case the one line drops.

Fault Tolerance and Web Hosting

Fault-tolerant / fail safe Web hosting is an expensive exercises for web hosts. More than one power supply and very complex networking can help with reducy.

What is falt tolerance?

Fault tolerancy is basically a computer system or component that is designed to take the place of a faulty component and ensure fail safe service without major interruption.

Big web hosting companies got multiple internet connections to their server farms through different leased lines and telephone exchanges. 

Power can effect routers, switches, firewalls and servers

Disaster recovery plans are important and the the bigger web hosts got them in place.

100% and 99.99% Website Uptime / Availability

Like I said earlier that I have not encountered a 100% uptime guarantee from a web host yet. There are however some that offer 100% SLA - service leval agreement, but if their servers fail or your site goes down then they give you a percentage of your hosting fee back.

So that is not a 100% failsafe guarantee.

The major web hosting companies offer 99.99% uptime and if they also have more downtime then they will give percentage of your money back.

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It is a good idea to buy both your domain name and web hosting at one place. Lots of web hosting companies give you a FREE domain name when you sign up for web hosting. Get both below

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Monitoring You Web Hosting Server

It is a good idea to monitor your site for performance etc. 

There are lots of software and analytical tools that you can use. You can even get notifications on your cellphone when the web server or your site go down.