port number

port number

TCP/IP port

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 client number is how the network address translation (NAT), which typically resides in the router, determines which user to send back the responses to. Although that same client number is used simultaneously within thousands of organizations, each TCP/IP network keeps track (via NAT) of its own assigned numbers for internal use only. A "socket" is the combination of port number and IP address (see Unix socket). See NAT.

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.

We're Listening
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.


Retrieving a Web Page
To request a Web page from the Internet, port 80 must be open in the network firewall to let the request go out. When sending back the Web page, the port numbers are reversed, and the user's network accepts the external packets because the request was initiated by a user. The IP is the numeric address of the source or destination network (see dot address).
References in periodicals archive ?
First, signatures based on port numbers analyze the traffic by using the port number provided by the Internet Assigned Numbers Authority (IANA).
When plugged into a network port, the CDP/LLDP data is automatically displayed on the home screen so users can quickly see the brand name, switch ID/MAC address and switch port number; providing at-a-glance access to critical diagnostic and troubleshooting information.
The destination port number of packet_i is the same as the source port number of packet_j.
With additional LLDP port decoding, this gives system integrators the ability to instantly see port information on the tester's screen, including port number, VLAN, MAC address, description, IP address and more.
- Port number to which other TCP/IP clients connect when the converter acts as a TCP/IP server
i.e., the smaller of the source port number and the destination port number.
Bandwidth control by port number is sometimes difficult because applications can be designed to use random port numbers to communicate and notto be recognized by traffic shapers (Stewart et al., 2005).
Therefore, enterprises will normally go through an off-production lab testing and validation and likely demand a maintenance window to apply changes, and have an opportunity to roll back, in the event of a problem occurring due to misconfiguration of one of the nodes; this could be as simple as a type in a script or a wrong port number triggering a loop.
so to figure out the serial port number see the screenshot below.
The socket is bound to an IP address and port number so that the TCP layer can identify the application.