To check the availability of a web server you need to regularly monitor a few web pages on your site but also check other sites too (for comparison). If it is only your site that is unavailable then it strongly indicates a problem with the server or its connection to the Internet. Armed with this information you can ask the web host why they are not achieving the high server availability they are claiming.
You may also check up on availability by Site Vigil's traffic scan. If the web site log does not record any traffic for a certain length of time an alert is generated and a report made. The following screenshot shows a number of 'gaps' in the web traffic that reflect periods when the web server was out of action.
Site Vigil keeps a summary of the monitoring results with all sorts of useful information available at any time, it contains an access time distribution graph as well as details of average access speeds, download rates and recent monitoring events.