Editing Port scanner (section)

==TCP/IP basics==
The design and operation of the [[Internet]] is based on the [[Internet Protocol Suite]], commonly also called [[TCP/IP]].  In this system, network services are referenced using two components: a host address and a port number.  There are 65535 distinct and usable port numbers, numbered 1 … 65535. (Port zero is not a usable port number.)  Most services use one, or at most a limited range of, port numbers.

Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host.

The result of a scan on a port is usually generalized into one of three categories:
#''Open'' or ''Accepted'': The host sent a reply indicating that a service is listening on the port.
#''Closed'' or ''Denied'' or ''Not Listening'': The host sent a reply indicating that connections will be denied to the port.
#''Filtered'', ''Dropped'' or ''Blocked'': There was no reply from the host.

Open ports present two vulnerabilities of which [[System administrator|administrator]]s must be wary:
#Security and stability concerns associated with the program responsible for delivering the service - Open ports.
#Security and stability concerns associated with the [[operating system]] that is running on the host - Open or Closed ports.

Filtered ports do not tend to present vulnerabilities.