About this test: In order to translate a mnemonic name (such as "netalyzr.icsi.berkeley.edu") to an IP address, your computer contacts a Domain Name Service (DNS) resolver. This is a computer controlled by your ISP which acts as an intermediary.
It is often desirable for computers to be able to query specific DNS servers on the Internet directly, instead of relying on the default server assigned to your network. Such queries may be blocked by firewalls or NATs which disallow the use of arbitrary DNS resolver, or which attempt to proxy DNS traffic while not fully supporting the DNS protocol.
In this test, your computer conducts a series of custom, direct DNS queries to our own DNS server to determine if some of these specialized queries are disrupted by the network. Since all these queries conform to the DNS protocol, any blockage indicates either a deliberate restriction on DNS or, more likely, a network device that attempts to understand DNS traffic without fully implementing the DNS protocol.
What if this test reports a problem: The most likely cause if some DNS requests are allowed but these specialized requests are blocked is an error in your firewall or NAT. Thankfully it is reasonably unlikely that your Internet experience is affected greatly by such broken devices. If you are on a public network there is not much you can do. On your home network, try updating your NAT to the latest firmware. If you are on a corporate network, the responsible devices should probably be updated.
For additional information on the results, please consult the Netalyzr help site over at the New Scientist.