This paper reports the most important techniques used by TCP port scanners . TCP port scanners are specialized programs used to determine what TCP ports of a host have processes listening on them for possible connections. Since these ports characterize, in part, the amount of exposure of the hosts to potential external attacks, knowing their existence is a fundamental matter for network and/or security administrators. Moreover, as scanners are also used by hackers, administrators need to know how they work and what possible weakness they exploit to be able to prevent unwanted scanning or at least to record each scanning attempt.