The designers of the original Berkeley UNIX "Sockets" interface, upon which much of the technology and practice we use today is based, set aside the specification of "port 0" to be used as a sort of "wild card" port. When programming the Sockets interface, the provision of a zero value is generally taken to mean "let the system choose one for me". Programmers who specify "port 0" know that it is an invalid port. They are asking the operating system to pick and assign whatever non-zero port is available and appropriate for their purpose.
As a result of this programming convention, there has traditionally been no way for Internet Sockets programmers to generate or receive "port 0" Internet traffic. So port zero was set aside and never defined or used. Although times and technology have changed dramatically, port zero has remained something of an unexplored "no mans's land".
However, with the widespread and growing availability of operating systems offering the "Raw Socket" programming interface which provides the means for deliberately generating port zero packets the presence and security of "port zero" is of growing importance.
The "Port Authority" revision to GRC's ShieldsUP! services and NanoProbe technology offers the generation of port zero probes to enable users to verify, secure, and stealth their system's handling of these potentially troublesome Internet packets.