A CDE Definition
A number assigned to user sessions and server applications in an IP network. Port numbers, which are standardized by the Internet Assigned Numbers Authority (IANA), reside in the header area of the packet being transmitted and thus identify the purpose of the packet (Web, email, voice call, video call, etc.).
Destination Ports Are Server Applications
Destination ports may be "well-known ports" (0-1023) for the major Internet applications, such as Web and email. For example, all port 80 packets (HTTP packets) are directed to and processed by a Web server. User "registered ports" (1024-49151) are assigned to applications that are mostly vendor specific, such as Skype and BitTorrent. See well-known port, port forwarding and opening a port.
Source Ports Are the User Sessions
The source port is a next-available number assigned by TCP/IP to the user's machine. This assigned number is how the network address translation (NAT) determines which user to send back the responses to (see NAT). Although that same client number may be used simultaneously within thousands of organizations, each TCP/IP network keeps track of its own assigned numbers for internal use only. A "socket" is the combination of port number and IP address (see Unix socket).
Reverse Numbers for the Trip Back
In the response from the server, the port numbers and IP addresses are reversed. The packet's destination port becomes the unique source port number assigned to that user's TCP/IP session.
TCP/IP servers are said to be "listening" for their port numbers to know when to accept incoming packets. If a human action had to be chosen for this, "looking" would have been more accurate. People "listen to sounds" but "look" for data; however, computerese has never been known for clarity. See TCP/IP and NAT.
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