A port is a conduit to the Internet that computer software uses to contact its server. Such ports refer to the commonly known TCP/IP ports, named after the Transmission Control Protocol and Internet Protocol. When a port is open, this means that the specific port is configured to allow transmissions between the software and its server, and accepts packets from outside sources. A closed port is the opposite, ignoring and not accepting any packets that may be transmitted to it.
Its inaccessibility is not the only feature that defines a closed port. A closed port is considered such not only if it is unreachable, but also if there is no software listening on that port. Software listening on a port means that there is an application able to receive the transmitted packets and recognize them. When there is no application listening on a port, packets directed to that port are automatically rejected by the operating system of the computer in question.
Firewalls may used to close ports. The user needs only to configure his firewall to allow specific packets for certain ports through, while other ports can be considered closed for all purposes as no packets will be allowed through. In this manner, unexpected packets which may or may not come from malicious sources will be ignored and dropped by any closed port.
Numbers, called port numbers, identify each port. By common practice, certain port numbers are reserved for use by specific types of services. Unused port numbers are usually closed for security reasons.
Serving as gateways between installed software on the client computer and the server, ports can also serve as pathways for malicious attack. Unscrupulous individuals can use software to scan open ports in computers they detect on the Internet, and to detect any listening applications on those open ports. When finding such an opening, they can take advantage of the open channel to obtain confidential data, sabotage the target computer, take control of it, or other illicit activities.
Another vulnerability of open ports is the possibility of either authentic services being wrongly configured or downloaded software being less than benign. Malware disguised as helpful software may act as a service, listening on a port in order to allow hackers access to the target computer. The hackers then need not search for such unprotected computers, and can rely on the malware to lead them to such openings. A closed port is therefore the key to defeating such attacks, both from malicious software within the computer and attackers from remote connections.