People get very upset when they go to visit Amazon, Netflix, or just their favorite gun blog and the site is down.
This happens when a site is not configured with high availability in mind.
The gist is that we do not want to have a single point of failure, anywhere in the system.
To take a simple example, you have purchased a full network connection to your local office. This means that there is no shared IP address. You have a full /24 (255) IP addresses to work with.
This means that there is a wire that comes into your office from your provider. This attaches to a router. The router attaches to a switch. Servers connect to the server room switch which connects to the office switch.
All good.
You are running a Windows Server on bare metal with a 3 TB drive.
Now we start to analyze failure points. What if that cable is cut?
This happened to a military installation in the 90s. They had two cables coming to the site. There was one from the south gate and another from the north gate. If one cable was cut, all the traffic could be carried by the other cable.
This was great, except that somebody wasn’t thinking when they ran the last 50 feet into the building. They ran both cables through the same conduit. And when there was some street work a year or so later, the conduit was cut, severing both cables.
The site went down.