This is a best attempt look up an doesn't guarantee to be able to look up the domain name for an IP address. In particular it will fail for most dynamic IP addresses (such as home dial ups) and will often give the wrong result where a domain is hosted with other domains on the same IP address (virtual hosting). This is not a failing of the service but a result of the way DNS works.
If the domain can be reverse looked up we will also try and determine if it is currently up. This is done by "pinging" the IP address. Due to firewalls and other network configurations the ping may fail and report a domain as being down when in actual fact it is up. The time out is 2 seconds this method is not terribly accurate and is here more for interest.