About this test: In this suite of tests we determine basic TCP connectivity available from your computer. In particular, we test whether your machine is allowed to send the following to our server:

• Manually crafted HTTP connections. We require these for a number of HTTP-related tests.
• SMTP connections. This tests whether you can send email directly to recipients. While this may be convenient for users, it is often restricted in order to limit spamming from infected machines.
• TCP-based DNS requests. These provide an alternative way to conduct DNS requests, but may be blocked.
• Other services. Here we test whether various common services (such as SSH, IMAP, or POP3) are blocked.

What if this test reports a problem: This depends on the protocol involved and the context. In some cases, the reason is merely technical and not a cause for concern. For example, many NATs will disrupt our FTP test due to the unique nature of the FTP protocol, but this does not stop FTP from working. Services are also frequently blocked for policy or security reasons. For example, many ISPs block the common Windows ports (SMB, RPC, etc.) in order to shield their customers from attacks, and restrict outbound SMTP reduce the volume of spam sent from any infected clients' machines. Corporate networks often impose a considerable amount of filtering as part of their network usage policy.

For additional information on these tests, please consult the Netalyzr help site over at the New Scientist.