Everyone understands that website downtime has a negative effect on business. The impact can be minor if the downtime is limited and the affected 
website is associated with a less significant part of the business. Else, the impact can be catastrophic if the downtime is extended and the affected website is critical to business operations, brand image, etc.
Not everyone understands that the accurate detection of a website outage is a complex process. If you verify that your site is functioning properly only by bringing it up in your browser occasionally, a day comes when you will be under the false impression that everything is working normally, when in fact no one on the internet will be able to reach your site. Even if you use local software continuously to check if your site is up and running, there are different ways when you can still be down and are not aware of it.
Some problems include:
1. Local Access: Depending on where you are physically located in relation to your web server, and depending on how your local network is implemented, you may be accessing your site without ever venturing onto the public Internet. In other words, your connection to the Internet could be completely down, yet you can still access your site.
2. DNS Caching: The domain name system (DNS) allows visitors to access sites by name instead of having to remember their numeric address (IP 
address). But that means every time someone types an address into a browser a service somewhere has to look up the numeric address for that name. These look ups can be cached at several different levels to keep the domain name system from being swamped by millions and millions of requests from users all over the world. This is a very efficient system; it causes a very significant problem if you try to monitor your own website locally: Your DNS server, like any other server, can go down! Depending on DNS configuration for your domain, DNS information could be cached for hours. Potential site visitors around the world will be unable to access your site because they do not have the information cached, but you will think everything is working just fine because your browser, or operating system, or Internet Service Provider (ISP) is using the cached information to successfully resolve the address.
3. Alerting: If you are using local software to monitor your website and you lose your Internet connection how will it send you an e-mail if it detects a problem? What, if your e-mail server (SMTP) goes down, how will your monitoring software contact you?
These are just a few of the many problems you may encounter, if you try to monitor your own websites with a local (internal) solution. The only way around these problems is to use an external website monitoring solution.
1. Website checks will occur from a location outside the local network. If access to your site is disrupted at any point within your own network (router failure, etc.) or within the network of your ISP, the problem will be detected.
2. A good external website monitoring service will not rely on cached DNS information to access your site. Each website check will generate a new look up, therefore, problems accessing your site because of a DNS problem will be recognized immediately.
3. An external monitoring service will still be able to contact you even if your own e-mail server is down provided you use an address that is not tied to your own network. The best services will also be able to contact you by SMS, or yet better by calling you on the phone.
It is critical that you minimize website downtime as much as possible. You cannot correct a problem until you know about it. Use an external website monitoring service to make sure you are notified about all outages as soon as they occur.
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