TCP/IP Basic Knowledge
The design and operation of the Internet is based on the Internet Protocol Suite, commonly also called TCP/IP. In this system, hosts and host services are referenced using two components: an address and a port number. There are 65536 distinct and usable port numbers. Most services use a limited range of numbers.
Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host. See: List of TCP and UDP port numbers.
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 administrators 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.
Read more about this topic: Port Scanner
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