Wikipedia
Netcraft is an Internet services company based in Bath, England.
Netcraft provides web server and web hosting market-share analysis, including web server and operating system detection. In some cases, depending on the queried server's operating system, their service is able to monitor uptimes; uptime performance monitoring is a commonly used factor in determining the reliability of a web hosting provider.
Netcraft also provides security testing, and publishes news releases about the state of various networks that make up the Internet.
The company is also known for its free anti-phishing toolbar for the Firefox, Internet Explorer, and Chrome browsers. Starting with version 9.5, the built-in anti-phishing filter in the Opera browser uses the same data as Netcraft's toolbar, eliminating the need for a separately installed toolbar. A study commissioned by Microsoft concluded that Netcraft's toolbar was among the most effective tools to combat phishing on the Internet, although this has since been superseded by Microsoft's own Internet Explorer 7 with Microsoft Phishing Filter, possibly as a result of licensing Netcraft's data.
The service can only process public IPv4 servers at the exclusion of IPv6. The browser extensions will display security information for a domain’s IPv4 servers even when the user is connected to a different server over IPv6.