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In security parlance, the term '''open port''' is used to mean a [[Internet protocol suite|TCP/IP]] [[TCP and UDP port|port]] number that is configured to accept [[packet (information technology)|packet]]s. In contrast, a port which ignores all packets directed at it is commonly referred to as a "[[closed port]]".<ref>[http://www.pcmag.com/encyclopedia_term/0,2542,t=open+port&i=48464,00.asp Pcmag.com encyclopedia term]</ref>
Ports are an integral part of the Internet's [[Internet socket|communication model]] — they are the channel through which applications on the [[client (computing)|client computer]] can reach the software on the [[server (computing)|server]]. Services, such as [[web page]]s or [[ftp server|FTP]], require their respective ports to be "open" on the server in order to be publicly reachable.
The above use of the terms "open" and "closed" can sometimes be misleading, though; it blurs the distinction between a given port being reachable (unfiltered) and whether or not there is an application actually listening on that port. Technically, a given port being "open" (in this context, reachable) is not enough for a communication channel to be established. There needs to be an application (service) listening on that port, accepting the incoming packets and processing them. If there is no application listening on a port, incoming packets to that port will simply be rejected by the computer's operating system.
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